Yes, if there is a possible scramble there. Is probably a good idea sometimes.
If you are landing a unit amphibiously, yes it would take the territory unopposed. But you have too clear the Sea Zone first, of course. You fail and the unit stays on board.
You can go that route if you want but don’t be surprised if it just helps you smash the axis even more. Russia is already hard enough to crack if a smart allied player is in control of it. If you give it five more points a turn for just declaring war on Japan it just stacks the odds more in favor of the allies.
Or, you could look at it as the Japanese player has to take at least 5 far eastern tt from the soviets to balance things out, because the Soviets may be a tough nut to crack with a smart player, but a smart Axis player should be able to find way around it 🙂