Depends on the fabric for me. I've been working with panels and handkerchief cheater squares lately and didn't wash those so they'd be easier to cut on the straight lines. Still working on getting these done so the jury is out as to whether or not this was a good idea.
Recently I found a fat quarter bundle on sale but debated about buying it since the colors looked so drab. Got it anyway and after washing the colors brightened up to just what I needed. Must have been really dusty from sitting around for a while.
I always prewash reds and other bright colors that are combined with white.