Not heresy at all. I use sheets for backing all the time, except I get them at the thrift store or from my used ones that are looking rough around the edges. I even use the vintage blends with no problems. (Audible gasp from the quilting police). I hand quilt and avoid the high thread count sheets. I made some small test samples, hand and machine quilted them and threw them in the washer/dryer to test. Nothing blew up or shrank funny, so I figured I was okay. Cheap backing fabric and no extra seams to hand quilt through.