In my opinion, buying too many fighters is inefficient. Infantry are a better buy on defence, and tanks on both offense and defence as you can get twice as many of them. So long as I have 4-6 aircraft to trade territories, I mainly just buy infantry and a tank or two. I tend to keep about 2-3 artillery floating around too, to help with trades when my aircraft have higher priorities.
Granted, buying fighters forces the allies to purchase more surface ships to protect their transports, but in the face of a BB or fully loaded AC, such a battle is unnattractive for Germany. You might sink a few of their trannies, but you have also sacrificed your main offensive units. That takes a lot of pressure off Russia and opens the door for more aggressive incursions by the allies in the future.
I guess it comes back to my cautious approach to playing Germany. The allies have the ability to combine their strength when defending, but are forced to attack you individually. By focussing on defensive purchases (mostly inf and a few tanks) you will be able to hold out a lot longer.