Other than when entering a quilt in a show - you make the rules! I often use sheets, but I try to buy a 'lesser' quality sheet (a lower thread count). Top end sheets with very high thread count are pretty but very difficult to quilt. And, with quilts that are going to be heavily used, abused and not carefully washed (mostly the ones going off to colleges or first apartments), I have often used blankets for batting and sheets for backing. Much cheaper and abusable and I am not overly sad if they get lost or ruined.