I have one on my lower back, one on my backside-- almost my back, and one in the front...a more personal one.
It would be easier to cover one on your back than on your arm. Keep in mind that people will see it when you wear short sleeves. The ones on my back I like because I can show them to people if I want to but they're otherwise not seen. As for the other one, well, not many have seen it.