To me it's easier to be one way or the other.. I always pre-wash just to weed out that one fabric that could ruin my quilt.
If I wash all my fabric then when I get a piece out of my stash I know that all my fabric is the same. I do wash all the backing fabric and dry it, because it is such a large single piece. I am more afraid of that shrinking than a 2 1/2 inch square.
I know some people feel one way is better than the other, but I think it's more important to figure out which side you want to be on and do all your fabric the same way.
That makes sense to me, does it to you??