I would square up before I put on the borders.. but sometimes that can cause problems too
if I am off square and try to fix then one of my squares can look smaller than the others. The most helpful thing that I have found to do is to sew about 1/8" from the edge all of the way around. It seemed like a pain at first, but that really keeps the quilt backing from slipping out while I am sewing the back side of the binding down. (I use machine, I don't hand sew that binding one)