I measure the first distance across the middle and use that measurement to cut my border. Then I distribute any differences across the actual edge. Then I press, measure the opposite side and repeat the process. I have never had a wavy border that way (but did have waves when I tried it the other way).
When I miter, I cut the border piece longer by 2X the width of the border + a little extra. For the miters, I measure and mark the ACTUAL length of the center measurement and distribute any differences between the marking points. It works like a charm.