I haven't been to Joann in a few years. Their fabric is substandard, not quite as bad as Walmart, but bad enough. I think my time spent making a quilt is worth more than that.
Besides, the closest Joann to me is an hour away. I'm lucky enough to work at an LQS only 3 miles from my house! My boss can order anything I want and I get the employee discount.
All that being said, I rarely buy quilt backs. I find that even quilt shop quality backs are thinner than other fabrics. And, I often piece my backs so that they are more interesting, using leftover blocks and fabric from the front.