I always prewash...period. I have had bleeding fabric that I had to treat with retayne, I have had flannel that pilled up so bad it wasn't usable, and I have had fabrics that shrank so bad that durung the first wash after construction they pulled loose from the seam. I take no chances anymore...it just isn't worth the time or money to ruin a quilt the first time you wash it. I see posts at least once a month from quilters who didn't prewash and have problems.