I agree with Chasezzz - definitely cut off the selvage and then square if you can.
I recently worked with a panel for a playmat for my Grandson and it was really very wonky! I washed it, tried starching and stretching, added a very narrow border to try to square it up but it still had a section that would "bubble" up a bit in the middle and would not lay flat. I added a few more borders as best I could to try to get it straight and then took it to a Long-armer to stretch it on her frame and quilt it. She said that she thought is was stretched when manufactured during the printing process. It was very frustrating but it all worked out in the end - looks ok and bubble gone!
