I have for 2 reasons. 1st when I have my quilt done by a LA. I know that as they are stretched on the frame they may get a little pull at points. This leaves me room to straighten the sides. 2nd is if I quilt myself. I will have a backing that is just enough it may end up a little short on one area. Then I trim to fit the shortage. It all works out in the end and I am the only one to know