I always prewash. I recently purchased a red "quilter's cotton" fabric at Hancocks and it went from 44" down to 38" after washing and drying. I shutter to think what my finished quilt would look like if I hadn't pre-washed that fabric! In my opinion, there is just too much variance between fabrics to take a chance on not pre-washing. I always dry in the dryer because I figure that most people I give quilts to will wash in washer and dry in dryer.