maxdtex
Is a wash a substitute for pre-shading (do they accomplish the same purpose)?:
No, not really. The idea behind a a pre shade is that it makes the underlying paint look dirty. Or it makes the areas not pre shaded look more faded. The pre shade changes the tonality of the base color above it. Pre shading makes the paint look faded, worn out or dirty.
The pin wash brings out the panel line details by making them stand out a little bit. It doesn't really change anything about the base colors that its being applied over. You could pin wash a perfectly clean aircraft (no weathering) and it would still look very clean.