I also like to use good quality muslin in paper piecing if it's for a quilt or purse as it helps prevent the stretching of the pattern keeping it stronger too. I just sometimes trace the pattern on the muslin or put/lay the muslin on the paper. I also use it for piecing on the top, Yet I haven't used it for backing yet over the years...may have to try it someday.

Hobby Lobby muslin
 

I've bought wide muslin from them for years.

I back crib quilts, throws, etc. Can get a lot out of that piece.