When you make a rag quilt they recommend that you do not wash the flannel before so that it rags better. I have never tried it that way because I always wash fabrics before they get into the sewing room. But if it doesn't matter if it is laying flat or not, then perhaps the unwashed would work great in a string quilt. Maybe try a potholder size to see the exact results for the type of fabrics you've been given?