I have done both. Used fabric to match and pieced the backs as well.
I was thinking of going to a higher quality muslin and saving myself some money on some of the quilts I do. Not all of them, but some of the plainer ones would be fine.
I think you should do what you like! I was at a quilt show over the weekend and there were some beautiful quilts that had muslin for backing and I appreciated that because it did not draw you away from the original beauty: THE FRONT.