It depends. I just finished a charity kids quilt with muslin on the back, simply for the cost savings.
I often will use fabric from the sale rack at our LQS (still excellent quality) that doesn't necessarily match perfectly, but blends or adds interest.
If it is an heirloom quality quilt, I will use a print of one of the fabrics used in the quilt top.
I've also been known to piece my backs. I even have a reversible quilt, made from the same blocks on the front and back!