Although there is some conversation about the air war being broken or ineffective I have seen occasions where air does seem to make a difference in ground combat.
I have found in the mid to late war the most important aspect of handling the Axis air groups is keeping fighter experience and morale as high as possible. You can do this by manual upgrades and keeping your intercept high to shoot down as many Soviet planes as possible. Also keep your LW bases tight in case the Soviet player decides to bomb your airfields.
The first turn of the game is the best opportunity you have in the first few turns to shoot down Soviet planes before they scoot to the national reserve.
