I use sheets all the time for backings, all cotton, flannel and poly/cotton blend. If you are machine quilting, I don't think it matters. The all cotton sheets do lie flatter and don't pucker quite as easily as the poly/cotton sheets. I try to buy the middle of the road thread count. The 180 thread count sheets are sometimes coarsely woven or have a loose weave. You really need to feel them and check carefully. I've had $5 sheets that were great and $25 ones that were not (slippery and hard to handle). Some poly/cotton sheets after a lot of wear and washings may pill (those annoying little balls) but by that time, the top is probably starting to wear really badly as well. Happy sheet hunting as using sheets really cuts down on the cost of your quilt.