If it doesn't block back in to shape so that you have straight even edge, have you considered perhaps "going with the flow" so to speak and creating an gently curved bottom to the quilt? The bargello has those beautiful curves in the pattern, couldn't you curve the bottom border to match and compliment the curves in the quilt design? Just a thought, and before you did anything you could cut a curved shape out of freezer paper and lay it over the bottom of you quilt to get an idea of how it would look with a curved edge.