Question:Is it better to apply a wash....

if thats the way your doing it, i would probably apply it before drybrushing. just so u don't wash away any drybrushing

On the other hand if you apply it afterward, a very thin wash will blend the drybrushing into the base color and lessen the harsh contrast.

I normally do the wash before, but now I'm reversing the steps.

The norm is to wash before drybrushing, that way, any rough edges of the wash will be blended into a more natural shadow.

The drybrushing is to highlight the lightest shades on the surface of clothing folds and wrinkles. The was might not take the highlight away, it wouldn't make it light and highlighted anymore like it should be. I do wash, then highlight.