Like Tom says, the flatter the starting surface the more wash will stick, and tint the whole model. When possible, sometimes I put the first darkest wash on while the surface is still glossy and before applying decals. This just provides relief around molded on details without darkening the entire thing. Then decal, another glosscoat to seal, flat finish, and continue with directed washes. One of the biggest pitfalls starting out is too much darkening, when in fact the overall paint finish should be lighter (but with more contrast).
This only applies to custom-finish models where you have the luxury on pre-fading the paint mixture. For a factory finish, you have to fade with an airbrush then be very careful about where and how apply washes. E.g. instead of coating the whole model, use a small brush to paint over just details with a thick wash that doesn't run all over then remove excess with a second clean brush. Heavy airbrush fade only works for cars with white or delux lettering however, since dark lettering does not seem to fade as fast as the base coat.
