more thoughts on quilt backings
While I think pieced backings can look good when using an allover quilting pattern, I have found that they don't work well with custom quilting. I have tried them a couple times and it's nearly impossible to coordinate the quilted lines with the blocks on the back. For example, if you quilt in the ditch somewhere on the front, the quilted stitches on the back inevitably are crooked or slanted in relation to the blocks on the back.
I have used both 108" and 45" fabric for backing. The choice really depends on the pattern and color I can find. I tend to use mostly reproduction/civil war fabrics for my quilts and have difficulty finding wide backing in those prints.