Most quilt fabric is a thread count of only 160. Going with a high thread count sheet for a backing , is not always a good idea. The high thread count sheets will no allow the needle to pass between the threads , and the needle will puncture the threads in the weave. One of the reasons LA pros do not like sheets is the high thread count.. it s harder for the needle to pass through ... and if they make a mistake and need to take out the stitches there will be holes , that will not dissappear completley.
Sheets can be a good choice.. choose an all cotton 200 thread count or less. I find a 180 thread count works wonderfully... but finding 180 thread count all cotton these days is not so easy.