I have never used a sheet in a quilt! Even in the very beginning. Had pretty fabric to use and so just did not occur to me to do that. I have bought 200 thread count muslin on the bolt in 45, 90 and 120 inches wide that I have used to dye fabric and have often dyed a piece as just the right back for my quilts, but just never had the desire to use a sheet for a back.
It was much later that I heard that other people were using sheets at least some of the time and I just did not know why any one would do that!