I use them all the time, and mostly use sheets for backing fabric. Higher thread count sheets are more difficult to hand quilt through, and cotton sateen can be slick to work with, but I never have any problems using sheets. I think it is against the quilt police guidelines, but I frequently use vintage sheets, and I buy flat solid sheets at places like Ross and TJMaxx and use them for backing all the time. Have even made a couple of whole cloth quilts with interesting sheets.