I do for several reasons.
*Shrinkage. Even if its listed as pre-washed or its a batik that's been through so many dye baths and rinses (some quilters think that all the rinsing equals no shrinkage or bleeding and that definatly was not the case with a lot of my batiks and hand dyes).
*Excess dye. Lately a lot of the fabrics I've been using have had to have a Retayne bath to set in the dye. I put them in the wash with a Shout color catcher after rinsing and air drying just in case I didn't rinse it out enough after the Retayne bath (some of those fabrics shed a bit of extra dye during the bath).
*Its got this "new fabric feel" to it, but that's not how its going to be once the quilt is washed. So why not just start off with fabric that's already washed BEFORE you put it into the quilt? It gonna be washed anyways!
*You never really truely know where your fabric has been and fabric gets dirty. Think of all the people who handle the fabric before it reaches the store and then how many quilters feel it up before you buy it. On the more extreme side, there have been horror stories where someone bought fabric from a garage sale or something and later found th fabric had bugs in it. I buy a lot of things off eBay and definatly pre-wash just for the reason that I'm not entirely sure where its coming from or where its been. Particularly if I'm buying used band t-shirts for my quilt...it might be gently used, but for all I know it could of been previously worn by some sweaty guy at a concert. That thought makes me want to throw it in the wash.
In a nutshell, I'd just rather be safe than sorry and not possibly ruin a perfectly good quilt.