By measuring through the center of piece before putting on the last border, you will be able to cut down on the "wave" quite a bit.
The next thing to consider is the purchase of a walking foot. It is better at keeping the layers moving together.
Another thing to keep an eye on is how you are handling the binding or anything ... a lot of time, we get into a lot of trouble on the long runs of joining because we have personal control issues.
We pull the top layer on joining a border, too tightly. Lighten up a bit on your hand control. Even when one cuts fabric on the straight of grain, it can be pulled out of shape.
Another method is to lay the border on your piece and pin in place.
if you measure the middle, many times, you will find that the edge you are attaching the border to is longer ... not a problem .. now, you can use control issues! <G>
put the longest piece on the bottom and ease in the excess. do not try to stretch the border to the quilt, ease the quilt into the border.
If the binding is causing the rippling? Check the stitch length as you are attaching it.
Your length of stitch must lengthen out to accomodate all of the layers that the needle/thread is going through. You may need to lessen the upper tension also ...
Hope this helps <wave>