I use both cotton and flannel sheets on the backs of my machine quilted quilts all the time with absolutely no problems. I think it's strictly a matter of preference and end use. My quilts are made to be used and sometimes dragged off to camp, sleep-overs or college. The sheet backings stand up to repeated washings and I don't have so many $ invested that I cringe when I see how wet and muddy my son's quilt is after his camp-out on the May 24 weekend. On the polyester thread note, we had the Superior thread educator for all of Canada at our guild meeting last night and there is absolutely NO REASON not to use it in our quilts. Use thread your machine likes and CHANGE your needle every 8 hours or with every project.