I have used sheets as backing and found them hard to quilt through (hand quilting). Should have washed them a few times first.

I have used walmart sheets as backing in several quilts. They are great,but I machine quilt

I've used sheets many times (hand quilting) and prefer them to expensive materials as I usually prefer solid colored backs for my quilts.

When I was newly married, I used to make inexpensive curtains out of sheets. One time, when they were on sale, I'm sure, I bought a comforter of a certain pattern, and then sheets enough for: sheets for the bed, sheets for curtains, and sheets along with a coordinating solid fabric for a lightweight quilt made of squares (I was still very new at quilting). It was nice to have them all matching for a change.

p.s. I also used a sheet for backing of my first quilt because it was wide enough to be one piece. I tied that one.