We had a sew-in at a LQS for Operation 1161 a couple of Saturdays ago.
As we were working, the shop's premiere quilter who works exclusively on the Gammill machine managed to quilt three quilts ... and someone was surprised to see a cotton bed sheet going on as the backing for a nice quilt.
Bed sheets, are harder to HAND quilt through, so that is why many say not to use them -- harder to quilt through because they have such a high thread count.
I had a foundation pieced strip quilt that I didn't want to add a batting to, so I chose to use a flannel sheet for the inside. Keeping in mind that I had used a REALLY old sheet as my foundation, I put the flannel in as batting, and then I figured it was time to push the envelope a bit and chose a flannel sheets as the backing.
I used Superior thread and didn't have a bit of trouble getting that lap robe quilted. And, my quilting machine is just a Babylock Quilters' Pro ... just a bit bigger table top machine!
A sheet is not a problem ...