I'm not that picky. I use whatever is on clearance at the quilt shop if it is to be quilted. If it is to be tied, I usually use Joann's Snuggle Flannel on the back. Never used sheets or muslim - if I'm going through the effort of quilting it, I want it interesting on both sides. Of course, until recently my machine quilting wasn't all that great. Much improved now. Once in a while I look for fabric that seems related to the front - for instance, I bought some pirate fabric to go with the pirate front. Still have to piece the back because they didn't have enough. But it will look interesting!!