Sounds like a lot of quilt police on here! I agree that measuring the length of the middle of the quilt and adding borders that exact length is more accurate. But... I've had wavy borders that way because I had to "ease" them to fit.
I usually cut my borders about 6 inches too long, lay them RST flat on the quilt, and pin every few inches from the middle to the ends, never from top to bottom. At least that way I'm close....
Tell the quilt police to take a hike and do it your own way!