Sometimes the weave of sheets is too tight and hard to quilt through. My niece always likes to use sheets when I do her quilts. I just make sure I wash and dry them to get out all the sizing and loosen up the weave. They are %100 cotton, but inexpensive. I think the lower the thread count might be better. One of the first quilts I ever made, about 20 years ago I suppose, I used a flannel sheet for the backing. I don't think it was 100% cotton, but it has held up very well. It was tied, not quilted, not sure if that makes a difference.