I use a method that Bonnie Hunter suggests on Quiltville:
http://quiltville.blogspot.com/2005/...der-hints.html
Instead of measuring the quilt then measuring the borders, you lay the quilt out, lay the border fabric out on top of it (middle), smooth the fabric out and cut it. Measuring tapes can stretch as well as fabric stetching. When I am doing borders longer than the bed or table I am working on, I smooth out as much as I can and pin, then move the quilt and do the rest. I mark both the quilt and border into 1/4s and line them up at those points. I use lots of pins, every few inches at least. I never have wavy borders using her method. I don't usually use the walking foot to attach, but it could be helpful. Whenever I can, I cut my borders by length of fabric (don't care for them pieced).