I usually go by the 7 hitpoints minimum rule of thumb, so if Russia is collecting 28 ipcs or more, then I will consider buying additional fighters. The received wisdom for light trades is that when you really need your Russian fighters and artillery to hit they will probably miss, and when you really need that Axis infantry to miss they will probably hit hehe, so an extra fighter may come in handy if you can afford it and it suits your playstyle. The problem, especially in rounds 3-5, is that if the Axis stack a big armor wall and start spreading fodder around, Russia can start hemorrhaging infantry and artillery pretty quickly just trying to block all the blitz paths on Caucasus or Moscow. If you put too much into the light territory trading, then it becomes harder to deadzone vs a big push. But sometimes you really need to prevent the Axis tanks from driving forward, and sometimes a bad dud on a light trade can upend your whole front line, forcing you to withdraw or hold when you’d rather advance. Ukraine, Arch, Belo all play that way on the German side, Kazakh or Novos on the Japanese side. Artillery gives you the most bang for your buck, clearly, but an extra fighter can also work if Axis aren’t already on top of you.
With 28 ipcs though you can also buy 8 inf + 1 artillery, or 4 inf +4 artillery, 6 inf + 1 art +1 tank, or even 7 artillery, which are all stronger all around buys than 6 inf 1 fighter. So it really depends on how many hitpoints the Axis are throwing at you and whether the British or Americans have any fodder/air to help you cover the core.
I find you can usually tell by the second or third round whether your Axis opponent is more inclined to consolidate and slam or spread their hitpoints around for the slow bleed. If they are consolidating then you probably want to do the same and spam artillery for a big wall so you can match their stacks, but if they are spreading heavier then you might consider aircraft so you can run more attacks per round and try to trade with less cost in hitpoints over the long haul. A good tell is watching to see whether your opponent is just trying to match you on the ground hitpoint for hitpoint in the air trades and being all conservative, or if they are sending more ground hitpoints than you have defending to try and push the fronts and be all aggressive in the trading game. So like say you have 1 infantry unit defending in Arch, if Germany is attacking with 1 inf +air they are probably the sort who trades light and favors consolidating. If they are attacking with 2 or more inf +air, then they might be the sort that favors the spread. You also want to see whether your opponent is conserving their artillery to deadzone, or if they are throwing it forward, because that is another tell for the opponent’s playstyle and priorities. So, say this time you have 2 inf defending in Arch, if Germany is attacking with 1 inf and 1 art + air, or just 2 inf +air and holding the artillery back to deadzone it might tell you more about who you’re playing against. Same deal keeping an eye out for the airblitz, like if you see that your opponent is willing to attack 2 of your defending infantry, with just 1 of their own + a shit ton of aircraft, that will tell you more about what you’re up against. If the opponent is particularly risk averse or prone to gamble, sometimes you can play off that, either by mirroring and trying to volley more predictably, or sometimes by doing the complete opposite to throw them off. Often you have to adapt to whatever the Axis are doing since they are usually in the driver’s seat, but if something went really well for team Allies, you might be able to dictate and get more brazen with your buys. Russia doesn’t have that luxury very often, but every now and again you can catch a nice break.
In the Gencon version, in A&AO at least, I think the choice for Russia is either come out swinging heavy in Ukraine if you’re willing to take a chance, or stackfest West Russia if you aren’t, but I haven’t had much success doing much else. Sometimes if the newer Allied player is struggling to manage the approaches to the center of the gameboard it can help to stick to a few hard and fast purchasing rules of thumb, for a couple rounds anyway, that might go something like…
For Russia buy more artillery and try to hold it back for a few rounds so you can build a big wall of it. Sometimes 3 artillery end up being a lot more useful than 4 infantry to manage your deadzones whatever your income level. Even if the instinct to stack infantry is strong, just trying to get whatever number of art pieces the remainder of ipcs will allow you to fit in when dividing out by 3s for max inf may not be enough to deal with a bigger Axis push.
For UK buy more fighters than you probably think you need hehe, mainly because they allow you to skip around between Europe and India more easily, and you will need a bunch to cover vs German airblitz of your eventual Atlantic transport fleets, to threaten an Airblitz vs Japanese transports from the center, or just to hold the line on Defense. W. Russia and Caucasus are optimal because they allow you to threaten Germany 3 moves away, while still being close enough to pinball around between India and the Canal, or China or the Eastern front. The more you have the easier it is to split for double duty.
For USA buy more transports and bombers than you think you need. If you buy a bomber per round for a few rounds, and don’t throw them away on early bombing runs, usually you can get something going before too long once you have a nice pile of them. Also when you transport your infantry around try positioning it where it can threaten two or more territories the following turn on amphib assault, instead of using them to trade territories right away. This is where having more transports and more bombers really comes in handy, because it forces the other player to split defense or possibly withdraw, without having to actually attack. Often you can stay in place and send the transports back for a repeating launch. When the infantry is already in place its easier to set up a stronger double punch with a 2 turn set up, rather than gunning early and allowing a round of delay for Axis to regroup before you can threaten again.
In general the more stuff you have in the middle of the map the better, so like a big party with everybody all up in Russia, because its usually easier to coordinate and find an opening from there than it is from either side.