If there is one long seam on the backing, I try to load the quilt with that running horizontally. Sometimes that means loading the quilt sideways, but that seldom is a problem (I don't have many directional pantographs, and I can custom quilt in just about any direction). If I'm forced to load a vertical seam (a 4-patch backing, for example), I just take extra care in rolling the backing onto the bar, to ensure the fabric is evenly distributed to whole way and doesn't sag at the end. I'll re-roll the backing several times to take care of this. Fortunately, I have not noticed issues on the take-up side in these cases (I do have a dead bar)