Remember if you are doing a multi colour camo pattern, you can always touch up the pre-shading prior to application of the top coats.
Obviosly hard edged, particularly Luftwaffe camo patterns are easier to pre-shade, but it can still be done with soft edged patterns, it just takes a little patience and planning.
I always apply a panel wash, even with pre-shading, especially on those panels which had a tendency to acquire a healthy deposit of dirt, ie behind exhausts etc.
One of the problems with pre-shading is decals, the rest of the paintwork looks faded but not decal markings, therefore an element of post shading is always required, either with paint or pastels. Best bet is to apply an overall 'filter' coat in a light shade such as Tamiya Buff, thinned about 90% thinner to 10 paint over the entire model. This knocks the brightness back on the decals, then follow up with a similar mix of darker shades, I use Gunze Soot & Rust, about 8 parts Soot to 2 thinned to the same ratio as before. Apply this mix with your AB to those panel lines which bisect decals until the shading matches that of the surrounding panels.
I sounds a lot scarier than it is. With a little practise it becomes second nature.
Karl