I've decided to no longer worry about the straight of grain on backings. I had a piece a few weeks ago that when I tore it (the way I usually find straight of grain) I lost 5" off one edge - had I torn the other side I would have ended up with a trapezoid. No way was that baby going to straighten out so I just used the fold as a guide and cut across. I quilt my own on a mid-arm and haven't really found any difference between squaring up with a rotary cutter to trying to follow the straight of grain.