I've hear of tattoos getting distorted as people lose or gain weight. Remember silly putty? Ever press it onto a comic in the newspaper then stretch it out to make the imprint all distorted? That's basically what happens to your skin with a tattoo.
I don't think that it would change much, if at all, on your back since there's probably not a whole lot of fat there to lose so your skin wont change. I would call up a tattoo shop and ask them that question though to get a firmer answer since they deal with it all the time.