Eric, in a nutshell the washout in theory will help make the inside of the wing stall before the tips stall. This is a sort of stability mechanism and the result is that when a stall occurs the nose drops straight ahead. Contributing factors are also wing loading, CG and airfoil loading. The first two are fairly easy as one should be building as light as the strength required will allow. For our warbirds 6.5 to 7.5 lbs is a reasonable window. If the CG is too far forward the additional elevator travel needed to compensate may create snapiness of the stall. The third one is less understood and that is as you pull hard around a turn the airfoil being used needs to maintain effiecientcy while being loaded with 25-30 G. If you have a light model with the CG placed correctly and have chosen a good airfoil you can eliminate the need for any washout. Everything is a compramise so the safest route is to add 2 degrees of washout and keep the airplane light.