It really depends on what America’s doing, if you ask me.
If America took out, or is about to take out Japan (by take out I mean completely neutralize, no fleet, no islands, no mainland but possibly they still have their worthless islands and Japan itself) and is building up ground units in SE Asia, I might say that the Russians should retreat to join the Americans and give them increased defensive position.
If it’s a traditional Kill Germany First game, I’d say suicide them against any German force you have a decent shot at wiping out, or doing serious damage too in the hopes of making the job easier. After all, if Moscow’s gone, odds are VERY strong you don’t have much in the way of real forces left. (Except if you were killing Japan and had a bad run of dice or something resulting in 13 infantry, 3 armor surviving way out East or something.)
But that’s just my personal opinion on the matter.
Also, if tech is enabled (ie it’s allowed in your game) you may want to consider your last purchase to go for Rockets and get your AA Guns out of Russia before it falls if you can. (At least the rockets can help the allies even if the enemy gets your armies completely destroyed.)