I always prewash, but that's mainly to get the chemicals out of the fabric. Our bodies most likely won't react to them if we only have a few yards of unwashed fabric around, but when we get a good stash going, all those chemicals can become a huge problem, not only for us (as quilters), but also for our family members, including our pets. Washing takes care of that problem (and many don't know that their symptoms (headaches, stuffy noses, scratchy throats, breathing difficulties, etc.) are caused by the chemicals in unwashed fabric).