It's kind of a "On the one hand...on the other hand" kind of thing.
When I think of ALL the fabric I've washed that did not bleed, I hate to commit to prewashing. But when I think of the small handful of fabrics I've washed that bled like crazy, and the quilt I made that was forever altered by bleeding, well, then, prewashing seems not so bad!
I play it by ear and live dangerously from time to time...