I always measure the center down the length of the quilt and use that number to cut my side border strips. The sides of a quilt have a tendency to stretch some from piecing and handling so the center is a truer number. After I sew the side strips on I measure across the center from side to side and use that for my top and bottom boarders. When I sew the border on I match up the center of the quilt edge with the center of the boarder and also match the quarter way points as well. This way whatever easing needs to be done to match the quilt with the boarder can be done as you go along. I never have wavy borders and when my quilt is done it hangs and folds straight. You can also take several measurements and average them together.