I am an almost totally hand quilter and I will not use a sheet for the back. I have enough trouble with my hands without trying to quilt on a sheet. I watch for when Joann's has their half price sale on their clearance fabrics and buy their better cotton remnants that have at least six yards. I have several boxes of backings in my stash just right for backing the quilting for hire I do. I might use a sheet for a child's quilt or something else small that I machine quilt just because it will wear well. My husband will not sleep under a sheet so I have a rather good supply of flat sheets for various projects just from the sets we purchase for our own use. This past Christmas I used some of them to fashion personalized pillowcases for my grandchildren. Each got one with a special embroidery on it just for them.
mpspeedy