Inspiring Quilters, Stitch by Stitch
I have read that sheets are not recommended because of the tight weave. Supposedly the needle does not go between the weave, but tends to break the threads of the sheet, leaving minute holes that will eventually get larger with wear and washing.
I used sheets many times. You need a smaller sharp needle than normal...12 , (or 10 if top has no seams, such as a whole cloth.) Some of mine are 20 + yr., and no sign of holes or bearding. However, if you didn' t alternate your seams on the top, you could break needle on a thick intersection. A lot of sewists piece the backing from leftovers to savd $ and makes the quilt somewhat reversable.
If you are hand quilting, stay away from high thead count, as these don' t needle well.
You can use sheets, but you will find it doesn't quilt as well as muslin. Just my opinion. Others may like sheets. I buy my bigger pieces from Fabric.com. They stock the 108 inch pieces for backing so you don't have to do seams. You can get flannel, muslin, etc. in that large a piece. Great for queen and king size quilts. No seams to deal with or trying to line things up straight on the back of a quilt.