Have to jump in here---as most of you know, a backing needs to be at least 6" longer and 6" wider than the top for longarm mounting. When the backing is so big that there are more than about 4" on either side (extra at the top & bottom doesn't matter), I trim off the excess BEFORE mounting the quilt. It is extra work for your longarmer when the backing is way oversize, and some longarmers charge for trimming; the whole problem can be solved by sending the correct size backing with the top! BTW, when I do have to trim backing, the long narrow strip ALWAYS goes home with the completed top.