Using a walking foot will really help. Don't pull to much on the binding, like the other said that may be what's happening. I always use cross grain and have never had a problem. I always hand sew the binding on the back, I tried machine stitching once and it just didn't look good and was harder to keep it from waving. Like FMQ, practice is key. Make some sample small sandwiches to practice everything including getting the corners right. Good luck