It depends whether you are playing with an Allied bid or not, assuming normal dice rules (rather than low luck) the difficulty with any Russian opening (even one with a bid) is that you have no control over how many hits the German defender in W. Russia will put up before being destroyed.
The swing is actually pretty significant. Its possible you might take no hits in this battle as the Russians, or only a couple hits, in which case your stack in W. Russia is probably safe. On the other hand you might take more than just a couple hits! A poor first round of combat can take you from a well defended eastern front, to a complete collapse of the eastern front almost instantly. I think the design of the first round battle in W. Russia, is the real reason people are bidding out Allies on this board.
I would suggest that setting up your round 2 attacks out of W. Russia, is just as important (if not more so) than simply setting up a round 2 defense of W. Russia against German attacks. In that respect a tank buy is more effective than an all infantry buy. Basically because the tanks can shoot forward out of Moscow, or Caucasus, against Karelia in the second round in one move (as soon as they are placed), making the Germans less likely to stack it in the first. An infantry stack takes 2 moves to put into position. Even the full 8 infantry buy might not be enough at that point to hold the line into the 3rd round, so what you give up in Infantry initially purchasing that tank, you might actually make up on the back end, just by keeping Germany 1 space further out with their stack than they would otherwise be.
Why Western Russia? Well the logic is pretty simple. Karelia is indefensible, the next best locations for launching fighters out of the UK are W. Russia and Archangel (both 4 moves.) Of these W. Russia covers more territory, since it can also be used to pressure Ukraine, and hold Caucasus on the counter attack. I would say if you are trying to stack W. Russia for defense, without taking another territory too, then you probably want 3 tanks and at least 1 gun at a minimum to protect your surviving infantry. An alternative would be to take Belo as well to knock down the total number of German infantry that can be brought into a counter attack on W. Russia, though that spreads you somewhat thinner, to the point where you might take too many hits in either battle. (You might even lose the Belo battle in which case it was pointless for blocking the Baltic tank!) Another alternative is strafe Ukraine, though that is a gamble. It is debatable whether you really need to hold Caucasus in the first round, or if it is better to put your fighters somewhere else. I think this will depend a great deal on whether you play with sz 16 open or closed. If sz 16 is open, Caucasus like Karelia is pretty much indefensible.
One way to deal with this is to try and spring a Caucasus trap. Basically you leave it light with Russian infantry for the defense, and try to draw the Germans forward before they are ready (putting either their tanks or fighters out of position). If they take the bait, you can then spring the trap in the second round. The British may be able to assist you with this, as they have 1 inf unit, the bomber (and possibly a fighter or two on Caucasus if need be) to retake Caucasus from Germany if the Germans go in too light.
The other big question to consider is the problem of Russian fighters… They only have 2! And where they land could make or break the game.
The Suez canal: In a no bid game, Russia is the only player on the board who can provide a defense of Egypt or Transjordan from a round 1 German attack. This is accomplished by sending the Moscow fighter to Egypt. If you really want to keep the Germans off the canal, it is also possible to reinforce the Egypt play by moving 1 tank to transjordan and landing the Karelia fighter in Transjordan on Non Com after it joins in the W. Russia attack. This puts both Russian fighters out of position in the second round, but could be worth it, if the canal is critical to your strategy. Doing both, pushes the odds for the German attacks on either Canal territory below 50%. Basically Russia does this to keep the British fighter in Egypt alive, and to give them a shot on sz 17 with the India fleet. It might be worth it, or it might not, depending on what sort of Allied game you are trying to set up. Clearly this move, means that you can’t land fighters in Caucasus.
Archangel: A German sea lion opening is very hard to pull off, no one will deny, but there is one sure fire way to prevent the Germans from even attempting it, and that is to land a Russian fighter in Archangel in the opening round. Some people will go this route, if they really want to deter a Sea Lion opening. But again, pulls at least 1 fighter off Caucasus, which makes it pretty hard to hold Stalingrad.
Szech, keeping the American fighter alive: to have much chance of this working you really have to send the Kazakh infantry unit to Szech. Even that might not be enough to deter the most aggressive Japanese player, unless it is backed up by a Russian tank, or a British fighter, or a stack on Bury.
Stacking Bury: This works best in conjunction with a strong India play, or a Pacific set up from UK. Its very hard to stack Bury for defense, even with 5 inf. Basically what you are doing is trying to tie down the Japanese transport and a few fighters, with the expectation that your infantry will die, but the sacrifice might set you up strategically. In which case it might be worth considering whether you can accomplish the same effect with only 4 infantry instead of 5? This at least leaves you 1 infantry unit (2 if joined with the Novos inf unit) that can be pushed towards Evenki and Archangel in subsequent rounds.
Karelia in round 2: how many tanks is enough tanks? I think this is the most challenging thing to get your head around. It is quite difficult, especially if you send a fighter to Egypt, to cover Karelia in the second round without buying tanks. A 6 inf 1 tank buy, gives you 5 total tanks on Karelia (provided you kept the 4th starting tank in range). A 4 inf and 2 tanks buy gets you 6 total tanks on Karelia. A 2 inf and 3 tanks buy gets you 7 total tanks. And of course a 4 tank buy, will get you 8 total tanks on Karelia in the second round. Note that only Tanks can reach Karelia in the second round (infantry are 2 moves out, which means they are only defensive until round three.) How many tanks are enough, and how many are too many? This will depend on the skill level of your opponent, and your own confidence as Allies.
There are two schools of thought on how to play Russia, one uses tanks to set up, the other stacks infantry for defense. Both can be useful when the inevitable arrives, and you have to start stacking Moscow for the red turtle final defense, but If you buy no tanks, you have no forward attack options. On the other hand, if you don’t buy enough infantry then you will get smoked in the end, when Germany and Japan start driving in force towards the center. It’s a double bind, with no easy work around. So just remember, whatever route you go, to send fighter support from the W. Allies!
No matter what you do, it is impossible to save the British battleship. Even if Germany sends all their airforce to the Eastern front, it can still be sunk with just uboats and the german cruiser. Though you might be able to keep the ships in sz 10/11 alive this way, the alternative is pretty crushing. In order for the German player to fly all his air east, that means he feels reasonably confident he can destroy your W. Russia stack on an even trade. This is what you want to avoid. You want to stack it large enough, that the thought doesn’t even enter his mind. Or else just pray for snow, and luck, and a monster defense hehe. Some games can be fun, if they start with a wild German offensive against W. Russia, others just end right then and there.
After a few games like this, you may decide whether some form of bid is helpful for the Allied player. Basically no matter what you do, the W. Russia battle is all in the defenders dice. Depending on what kind of game you like to play, you might just want to go aggressive. At least then, you will know right away, whether it will be a grinding uphill slog for the Allies, or something slightly less so. Hope that helps