I also would machine stitch your edges before washing. Either serger or regular machine. I think even just a straight stitch would be fine. You'll probably be trimming to straighten anyhow or at least have the stay-stitching in your seam allowance. ALWAYS wash your flannel. It was recommended to me when I first started quilting to wash everything in the hottest water you could and dry it on the highest setting so you get as much shrinkage out of the way before you start working on a project. I also recently heard from a quilt shop instructor (who sews everything from quilts to clothing and everything in between) that you really don't get the full benefit of the washing/drying process if you don't have a full load going into the dryer. If you're only drying a few bits of fabric the dryer is not running as long (unless you use a specific timed setting) and therefore doesn't get as hot. Actually made a lot of sense to me.