Also something that just happened to me,
Make sure your backing is always at least 6inchs extra on all sizes of your top, the fabric moves as you quilt. My customer had made a quilt back in the 90's and had the backing fabric from back then and there was only like 3inches extra on the backing all the way around. Needless to say we tried to make it work due to not being able to buy anymore of that fabric...well, by the end of the quilt it had shrunk up and we were 4 inches too short on the bottom....we had to trim off the extra on the sides and sew that onto the bottom to finish it out. So always make sure you have lots of extra backing because it isn't worth the stress and frustration if it happens to you...though I am told it happens to every long armer once but I wish it would have never happened to me :/ oh well.