I remember making my first quilt 20+ years ago and buying fabric for the back. I wanted my quilt to be reversible and the woman at the quilt shop was shocked that I was "wasting good fabric" on the back when "you're supposed to use muslin".
Now I rarely see muslin used for backings, and I am starting to be drawn back to that old look. I guess I just don't like to do what the mainstreamers are doing, lol.
Any way, use what YOU like, just use good quality fabric, whether muslin or otherwise so your quilt will hold up.