When I add my borders, I always measure, measure and measure. What is the carpenters rule 'measure twice cut once'! That should be a quilters rules. Measure in in multiple places, then take the average of them. Then cut, pin to quilt top, ends first, middle, etc, etc. Because I pinned it, the odds of stretching the fabric is reduced.
I'm sorry if I am 'preaching', but I have seen some beautiful quilts 'destroyed' because the quilter did not take the extra time, their borders are just 'waving' to everyone. Some times when we get to the border . . . it is like almost finished . . rush and it's done.
Good luck