I agree with Claire, I like it the way it is..... and I don't know if this would work but wouldn't it be pretty to leave the top and bottom uneven. And quilt it the way it is with the nice triangular shapes? ( I might have to try that one myself lol)

But if you don't want to straighten it, I think it would be easier to do after the sashings, again that's just my opinion, though.