I have never prewashed my quilt shop cotton and have never had a problem. I do wash any that I buy at yard sales or thrift shops. My question is, can you mix them in a quilt? I have kept them seperate in my stash as I was trained not to do that. I will wash the new fabric first if I need to combine them in the same quilt. So now I wonder if it wouldn't be easier to just wash them all .