I use sheets all the time, partly because I'm a cheapskate. But if I need some fabric NOW I often resort to using poly/cotton sheets since I live in a small island community that doesn't have much in the way of retail fabric selection. We have Joann's and a small LQS that I can't usually afford. I also wash my fabric before cutting and piecing so any shrinkage has already happened. I have never had any problems mixing cotton and poly/cotton sheets either while making the quilt or afterward. It's a great source of yardage especially on sale! Go for it!