I also stitch the binding on the back. My preparation then is to press the binding outward toward the edge of the project. Carefully stick the nose of your iron into the mitered corners. Turn the project over and press the perimeter all the way around again. You will note then how flat the edge is - no waves. And lastly, I miter the corners (and the overlap) and pin securely before adding a deco stitch. Fiddle with this miter until you are satisfied. Doing this is so much better and easier than doing it as you stitch and hoping it will be okay. --- Remember, judges will always look at your corners!
One other important thing: put on your walking foot. Use it to attach the first binding stitches all the way thru the deco stitching. This will help w/stretching or easing in any fullness of project or binding.