Assuming Japan doesn’t mess up horribly -
A fat Japan threat to Western US heavily slows early Japanese progress in Asia. Let them threaten. As soon as they move out of range, W US tanks blitz to E Canada, and W US infantry moves to W Canada preparatory to moving to E Canada for the E Canada-London or E Canada-Algeria drops.
If Japan sends its fleet anywhere but the Pacific region, US and UK both have starting subs that can harass Japanese shipping. True, a Japanese destroyer or two can greatly decrease the Allied threat, but early Japanese destroyer builds hinder their progress in Asia and Africa.
If Japan heavily commits its fleet to, say, Mediterranean, or sends around South America or Africa, then a few US units in Pacific can really screw with Japan. Later in the game, if there’s no Japanese navy or air in the region, US can start grabbing those high IPC islands with a transport (just 7 IPCs) while keeping off Japanese naval units with cheap subs. Japan would be forced to either bring a carrier or battleship back, or build destroyers/subs, plus possibly pulling back fighters to deal with the situation, at which point US could build destroyer/carrier/fighters in Pacific and increase the pressure on Japan. Once US has its infrastructure set up against Europe, it usually doesn’t need to invest much more expensive IPCs in naval for the Atlantic, freeing resources for Pacific harassment.
As far as US fighters/bombers moving away from Pacific, there’s two scenarios.
Japan hits Hawaii heavily: If Japan went to Hawaiian Islands fleet super heavy, you’ll probably see 1 battleship 1 carrier 2 fighters 1 cruiser 1 sub. (This is possible if sending 3 fighters 1 bomber to Hawaii and taking Japanese air as casualties). US’s counter of 1 battleship 1 bomber 2 fighters 1 sub (023344, 6 hits at 16 attack) attacking into Japan’s 7 hits (0123444, 7 hits at 18 defense) is not particularly favorable, even if you account for Japan having to sink its fighters before its carrier. Consider what happens with a bit of bad dice - the US battleship dies plus a load of expensive US air, the Japanese battleship survives, and possibly a Japanese carrier and even a Jap fighter.
Japan hits Hawaii lightly: US air doesn’t have anything good to hit in Pacific anyways.
Of course the Axis have some counter to everything Allies do, and vice versa. If there weren’t counters, then the game wouldn’t be particularly interesting; you’d just spam the attack for which there was no reasonable response, and you’d win every time.
IF Japan screws up or has super lousy dice and looks to lose at least 2 of its 4 battleship/carriers by the beginning of J2, then US can think about KJF (Kill Japan First). Otherwise, I’d send US air towards Atlantic ASAP to help in whacking the German battleship. Ideally, you want to whack the German battleship on UK2/US2 at the latest.