Absolutely agree - sheets are great for quilts. I often buy used top sheets in yard sales and use for batting in the center for a summer quilt or a couple of layers as batting in a baby quilt (holds up well to all the many washings). Sheets make wondering backings on a quilt, and I've used in place of muslin for blocks on the front of a quilt, too. As long as the sheet is in good condition and the thread count is high enough, go for it!