To be honest with you, I'm not sure how you'll be able to trim the edges accurately. Because the squares are coming together wonky, your rows aren't straight. When you take a straight edge to trim the overhang, how will you make sure that you aren't actually cutting at an angle? That would mess you up terribly when you go to attach rows. I don't know how you will be able to prevent that since the rows aren't squared up. If you did somehow find a way to trim them accurately, you would need to trim all of the rows, otherwise some of your rows will be made up of smaller squares than others. I hope that made sense.
That said, welcome to the board! I truly hope that you find a good solution. Once you have the top pieced together, quilting and washing will hide a multitude of sins. :P