Maybe I have been lucky. I have never averaged my length of fabric for borders and never noticed any wavy edges or pleats. I piece borders so they are a few inches longer than the measured side of the quilt top. I pin the border on from the center out to each edge and sew from the center out to edge turn the quilt and sew center out to finish one side.
I repeat this, doing the sides first and then top & bottom, ( This is my preference). I have never had problems that I cared about and if a quilting police were to mention anything about it I wouldn't care!
peace