Using sheets is 'hit and miss'. Sometimes they work just fine, other times - ick! If the weave is too tight, the threads break instead of parting for the needle, weakening the fabric. If a customer brings in a sheet, I tell them I'll try, but if it looks like the threads are going to break, I'll call and we'll look another option. The sound of that thread breaking is just like fingernails on a blackboard!
I have one customer that brings in flannel flat sheets for her backs. Those work wonderful!!