Coldquilter --- I've had that problem............and usually it's because of bias cuts in the borders. I always baste the left side as I'm going down. If I keep that in the line (using the painter's tape), then you just quilt and quit worrying so much. Mainly, cuz there isn't anything you can dow once you've started quilting. You'll be surprised when you get it off the machine -- usually you can't see anything was wrong or a problem! In the future, fold the quilt top in half lengthwise and hold it by the fold (you'll need a helper) and look at the cut bottom edge........is it hanging straight or is there a difference? Now switch and fold it widthwise and hang the same way. If it's hanging straight and doesn't show a difference in the width, then your quilt is okay. If you have problems when rolling, it may just be the stretching or the leadercloths may have a problem. Doublecheck them before the next quilt is put on.
PS -- as Ka9sdn said --- do doublecheck how tight you're making the side clamps. BUT, do NOT clamp them to the quilt top.......just to the backing and maybe the batting. I just did a charity quilt that the fabric was 2 yards for backing and 2 for the top. I added cheater cloth to the top and bottom so it could roll on but didn't think of the sides. I clamped both the top and the backing..............ended up with unwanted pleats on the back!!! DUH moment!!!