It is absolutely a MUST that you measure the top across the middle and cut the border to that length!
I pin at each end, then I fold the top and border in half and pin them at the center point. I repeat the folding in half and pinning at that point at least one more time. This helps distribute the fabric if there is a difference in length.
You are supposed to put the side that has too much fabric on the bottom and let the feed dogs help control the easing process.
It is really worth the measuring and cutting to fit because you won't have the waves. (I have taken the non-measuring shortcut and spent time reverse sewing.)