I prewash for one reason only, to remove the chemicals in the fabric they put in there to deter mildew. When I was a new quilter, I would store my newly bought fabric on open shelves in my quilt room, only washing it right before I was ready to use it. I was getting headaches and feeling generally queasy when in my quilt room. I soon realized it was the accumulation of all the chemicals in the air making me ill. I took a few days to wash all my fabric and within a few days I was feeling much better. I always wash my fabrics the minute I get them home now and have never had any more trouble. That's my two cent worth! :D