I use sheets all the time for backings, and have also pieced with some. Normally you would think of muslin as 100% cotton. Is the fabric content indicated? If they are all cotton, I think you are good to go, but even if they are a blend if you like them, use them, especially if you are machine piecing and quilting. If they are high thread count, it may make hand work difficult.
Here's a quilt I made a couple years ago. The beige fabric was a jacquard sheet...
http://www.quiltingboard.com/picture...es-t85293.html