The sheets I use for backing are 100 percent cotton, and I actually PREFER used ones to new. The older ones have been washed many times, any color loss has already occurred, any shrinkage has already happened, and now I can cut and sew it to my satisfaction. To me, the older ones are better made and seem thicker than most of the new ones, with the possible exception of some of the higher priced ones, and at least one of those I'v felt didn't seem that a high quality for the big bucks.
I use poly mainly for its long lasting quality, but for a very fine one, just don't know. Never made one that might be show quality, only for kids and friends and myself.