What I have read that has always kept me away from sheets was regarding the sheet's tight weave. Apparently since the weave is so tight your machine needle could actually be puncturing the threads and breaking them instead of sliding between the threads like on quilting fabric. I don't know this to be fact so don't quote me on it, but the thought of it scares me enough that I just buy muslin for backing on quilts that are okay with a plain back.