I'm actually not as concerned about bleeding as I am about the chemicals in the fabric. I know they say it's safe, but I'm not buying it. It irritates my eyes to be in a fabric store for very long unless it's one of those well-ventilated big warehouse places, and because most of our quilt fabric is imported, it's sprayed with pesticides when it enters the US. I don't have super-sensitive skin, but I really would prefer to wash out all the excess dye, finishing agents (especially the ones that make the fabric seem heavy and soft and "silky" - those seem to be the worst) and pesticides if I am going to handle the fabric much. They use a LOT of chemicals when they make and ship fabric!
Also, the fabrics shrink more than they bleed. When I was working with a new quilt shop owner, Benartex was sending out little "no need to prewash this flannel - it won't bleed!" cards to give customers. We washed some of it, and it went from 43" to less than 39". The Moda calicos that we tested shrunk to 40" of usable fabric (there was another inch or so of printed selvedge.)
I put too much time and energy into my quilts to risk it. I prewash EVERYTHING.
Oh, and I guess I do worry about bleeding, too. :-)