I always pre-wash. So many times, I've washed fabric and had it bleed and/or shrink. Doesn't matter whether the fabric came from WalMart's bargain bin or the LQS. Most of the time it doesn't but sometimes it does. Why take the chance of having a quilt ruined, if it can easily be avoided. Several times, the fabric never stopped bleeding, even after a dozen washes! Yes, literally - I counted! One is a piece of denim, which I set aside for a dog bed. The other was a lovely piece of blue, which I finally returned to the store.
When I finish a quilt and give it to someone, there's NO way I'm gonna burden that person, with special washing instructions! My quilts are meant to be used and enjoyed, not to be a PITB when it comes time to wash them.
Bottom line is pre-washing takes a bit more of your time BUT once the quilt is finished, you know it can be washed without worry.