I would also like to point out that in the craft, one of the biggest things we deal with is symbolism and association. For example if you ask me what the color black means, I would say darkness or evil, as I associate black with darkness. If you ask a different person they might say that to them it symbolises protection or the unknown.
You should consider the same principle when it concerns the tattoo, and which way up would it symbolise the correct meaning to you. I hope this helped and Blessed Be!