As a quilter borders are my kryptonite. I love piecing tops and seeing my final vision come to fruition. Keeping flat borders feels like trying to leap over tall buildings in a single bound. Last time I checked my superheroine costume was retired.
I would love nothing more than to cut a nice long strip, start at the end and sew like the wind. As a longarm quilter there is nothing that frustrates me quite like wavy borders. Wavy borders are created with my sew-like-the-wind attitude.
Wavy borders are created when the quilt top will has a more relaxed outer edge than the center which causes a flare. This is exaggerated when a quilt top has a lot of piecing in the body of the quilt which causes a tightness in the center, the outer edges will now measure longer than the center.
How can wavy borders be prevented? Before cutting the borders take three horizontal measurements, one on the top, one in the middle and one at the bottom. Here is an example. This top has different measurements at all three points. In order to get the correct measurement for your border add 62 + 59.5 + 61 = 182.5, divide that sum total by 3, 182.5/3 = 60.83. I know, what the heck, 60.83 call me crazy but a wavy border is looking pretty good right now. I round down by 0.125 to 60.70 and cut my border to the closest reasonable measurement which is 60.75
Rotate the quilt top and take measurements, including the borders, in the middle, on the top side and one on the bottom side. This will be the length you will cut the side borders. Again, you may need to work in some fullness.
The ultimate situation would be to have a square center before adding the borders. There are multiple ways to ensure that your quilt top is square while you are piecing it, square all blocks before sewing them together and checking to make sure you have an accurate and consistent .25" seam allowance are just a few suggestions.
By the way, I found a smidge of super power and my customer's quilt turned our beautiful, of course I had a beautiful quilt to work with.