IMHO I always measure each side and then the middle and I average them out and cut my border to that measurement. It's also best to try and cut your borders on the length of the fabric rather than crosswise. In a lot of patterns they tell you to cut your borders first. I used to have the same problem...I sewed my blocks nicely and squared everything up and it would still have those wavy borders. But this seems to have worked the best for me. Hope this helps. Using starch is another great idea. I do go thru a lot of starch.