Hmm. Well, personally I have never cared that much about my backing matching up, because most people only look at the front, but that's just me. I like scrappy backs.
Your next course of action depends on how you are going to finish it. No matter what you do, I'd wait to "square" it until after it's quilted or tied.
If you will be machine quilting it, or sending it to a longarmer, you don't have to square it, you only have to make sure you have enough backing to load it on a frame. Most longarmers request the backing to be 6" larger than the top, but if you're sending it out, be sure to check with your longarmer before trimming. I know of one longarmer who requests 8" on the top because of the equipment she uses. This number doesn't have to be exact, the quilt police won't come after you if it's 7" instead of 6". After it's quilted, then square the quilt up and bind it.
If you want to turn the backing to the front and bind it that way, I'd wait until it's quilted and then trim the backing to your desired width.
Does that help?