I usually do not prewash, but yesterday OPENED my eyes. I am starting a quilt that has adhesive in it for applique (usually hand applique, but no time this time) so I hand washed my fabric. So glad I separated the colors! The red bled so bad. The blue also bled a little. New fabric, fresh off the bolt, from an LQS. Had I used that fabric without prewash, the quilt would not be fit to gift! Since the "new" process of dyes, etc. I thought that the only reason to prewash was for shrinkage, therefore, rarely prewashed. I am now a believer!!