I used a sheet once. I was very new and also very poor, so I used what I had-- an old twin sheet that didn't fit the bed we had. It was poly and cotton. It didn't shrink at all but my all cotton brand new fabric top did shrink a bit. The batting bearded through the sheet backing where it was quilted. And oh, I was quite disappointed.
Now I use only cotton, all 100% quilt shop quality cotton, and I prewash the fabric for the top and the backing. I don't like surprises.