We do charity baby quilts w/one side flannel and the other cotten. I wash the flannel twice and only buy very good stuff...because some of it can pill, shrink, and just look yukkie after a second wash. I would think the edges might curl up on themselves even if they didn't fray. Sorry, can you tell it isn't my favorite fabric...lol. I've never done a blanket as you described. I do a decorative stitch around the edges after turning inside out and tie it in a few places with embroidery thread. Did 5 like this and went back to all cotten with flannel as batting.