Like Sharon B. says..it is a personal choice.
I always prewash because I don't want to take a chance that one of the fabrics I use will shrink more than another especially if I am using fabrics from different manufacturers. Many members on this board that prewash use starch when they iron the fabric to give it more body and to stabilize it.
If you use batiks or hand-dyes I encourage you to prewash or use a color catcher the first time you wash your quilt as often times they have excess dye.
If you decide to prewash clip off the corners of your fabric (a small clip will do). This will minimize fraying.