I'm a new quilter and have read all the opinions of the ladies and gents here on the QB and my LQS suggests I wash all fabric. Sooo, I do. Just my luck, I don't know much about fabric, Yet!, but I would be the one to make a fabulous looking quilt only to have it fade, bleed, shrink and whatever else fabric can do. So, I prewash no matter what size the pieces. Just my opinion!