I've never used sheets in a quilt, but I know there are people who do. The reason "they"say not to are the thread counts in the sheets as apposed to fabric. The high thread count sheets would be extremely hard to hand quilt through. My opinion is, if that's what you want to do fine. Unless younger putting your quilt in a show, a Quilter can do what they want. I mainly don't because I like my back to coordinate with the front and I don't care to use solids. There's been lots of discussion on the board over this very topic.