I only do it on a flat surface. The flat (matt) surface microscopic irregularities 'keep' a bit of the wash and tend to darken the whole color scheme. On a glossy surface, I would not do a wash per se, but rather apply selectively, precisely, minute amount of a diluted paint into the recesses (like the one you'd have between the wing and the door on a car body). In effect, this is applying a wash too. But when I apply a 'real' wash, on a tank for instance, I slap the thinned down paint everywhere!