Last week at the quilters retreat in WV we pieced the backing on the diagonal as instructions in John Flynn's web site.
It really works. Backing was 44" and quilt was 48" There is a formulae for the number of inches you need to add in order to increase the width. It took 20 more inches to add to the size of the needed backing. So instead of 66" it took 88". Sure saved $$ and could hardly see the seam. I recommend the method. The instructions are on John Flynn's web site.