I always prewash flannel, because it shrinks a lot, can shrink unevenly, and may also shed dye at first. Worst of all, some flannel pills, making it unsuitable for use on the front or back of a quilt. (It's still fine for the middle though.) I have had supposedly good flannel that did all of these, and cheap flannel that behaved beautifully. I use 3 layers of flannel instead of batting, so it doesn't have to be quilted at all. So far I have always washed and dried my rag quilts in my home machines, checking frequently for lint and strings. I have another one ready for snipping now, and since it's much larger than my previous ones I am considering taking it to a Laundromat.