Flannel is great on the back of a quilt. It is what I use on most of my lap quilts and baby quilts. I do not prewash the flannel as I do not prewash the cotton, and I haven't ran into any issues in the twenty-two years I've been quilting. I do however, wash the finished quilt in cold water, zero for delecates and 2-3 colour catchers. I find flannel is just so snuggly!