1/2" is nothing. Seriously.

Is it a decent quality tank? If so, I wouldn't worry too much. Just think about our poor friends in California that go thru earthquakes.

As long as the platform it is sitting on is consistent, you should be fine. I'm loooking at my 55 right now that's about 1/2" off, and has been for 5 years.

With all that being said, if it drives you a little nuts now, you should fix it now. A lot easier now than later.

BTW, we are talking about hydrostatic pressure. The difference is really nothing/