First, how far out of square is it? Have you quilted it yet? If it looks good on the bed, I would not worry about it. If you haven't quilted it yet, consider putting a border on it.
No matter how hard I try, mine are rarely perfectly squared. I put a border on all my quilts, usually about an inch wider than I really want it to be. Then once quilted I can "square up" the finished top. If the border is a tad narrower in one place it doesn't show and when the quilt is folded it is nice and square. Please bear in mind, I do not make show quilts and intend for mine to be used.