If it's a little bit off, you may be able to starch it or block it into place. A lot depends on how far off it is and why it is off. If it was cut on the bias and stretched, it may be easier to take it off.
Before I add borders, I check to see that the top is square, and adjust it until it is. I know it's an extra step, but has saved me from a lot of headaches later on. A walking foot for long runs, like borders, help, too.