Did you know that your local Wal-Mart has quilt backing fabric in a wide variety of solid colors (and some prints), in standard bed sizes, no seams, you can get a 90 x 102 for about $15!
The recent quilt I made was 80 x 90, so I needed backing fabric 88 x 98 (4" bigger on all sides for the quilting machine). I did my figuring and would need about 7 yards of fabric, which runs from about $4.95 to $11.95 a yard at a fabric shop, then I would have to cut and sew it together to reach backing size.
Or I could just get a sheet. Hmmm, let's see...
The front of a quilt deserves the good stuff, all that work and pieces and time, but the back? And the problem with the recent quilt I was working on, it was for a guy and most of the fabrics I looked at were girly. So I was already leaning toward a solid color anyway. And the quilting will show up very nicely on the solid. So, no-brainer - just get a sheet, wash it, trim to size, and send it along with the quilt top to the quilter. Voila!