I'm new to quilting, and have made several quilts. Each one had a different backing: used sheet; bleached muslin (pieced); fabric pieced together & 108" wide fabric (purchased on-line). I either machine quilted them or handquilted. They all turned out ok. No one complained/commented on what I used as backing. **Some people have told me to never use a sheet. But I couldn't get a good reason from them as to why not.
Have about decided quilting is a lot about expressing ourselves and enjoying the process. Whatever works for us, should be good enough.