It's just a matter of preference. I haven't used much muslin in a long time. But, with the economy, I think I will be using more in the front of the quilts. I sometimes use it for the back of wallhangings. My favorite is Kaufman Kona Mulsin.
I love pretty backings though. If I shop well, I can get decent prices on nice brand name prints for the back from online shops. A little more expensive than the Kona muslin, but still about half as much as the local quilt shops.