Some people say no, and others say go ahead. I think Jenny has put her stamp of approval on using bed sheets. Some are heavy, and would add extra weight to your quilt, if you are OK with it and can manage the quilting, I say, go ahead. You know the sheet is going to wear well, and some are really inexpensive if you can find them at a discount or on clearance. The only thing I have found is lately you have a hard time finding sheets sold separately and not in sets. The poly in the sheet may make it prone to pilling, that would be the only downside I can see. 100% cotton would be preferable.