It sounds like it's not enough to worry about. I'll often tighten the sagging section by twisting the rolled part of the quilt around the bar.
I always wish that backing fabrics were torn, that's the only time I like fabric being torn, but you are ensured of a square backing that way and the rolling back and forth is what pulls any "out of bias" from being on the bolt straight. Sort of like how we used to do in sewing class before we laid the pattern out :).