Using a flat sheet is a good idea, since the fitted sheet is what wears out first with a sheet set. However, not so good for hand quilting as I found out - a lot of them have sizing on that makes them stiff and hard to quilt through.
When we started making quilts back in the 1980's, a lot of people commented on how nice it was to see a pretty print on the back. I guess they were used to muslin.