I recently saw a woman in a coffee shop with om mani padme hum tattooed on her ankle (for more on this mantra, see my previous post here). There was nothing wrong with the tattoo, per se, but the placement is questionable. In many places in Asia the ground is considered dirty and ritually unclean. (This is especially true in India and Southeast Asia, but also applies in Tibet and other Buddhist areas.) Respected objects, therefore, are rarely placed on the ground, or even on low tables. Shrines and sacred images are often the highest objects in a room. The same applies to body parts, where anything below the waist is generally considered unclean or polluting. Particularly feet. Which makes feet not the best place to tattoo sacred symbols like the maṇi.
Some years ago in Nepal, I remember listening to one young woman ask a lama for help writing out the Kālacakra mantra, which she wanted to get tattooed on her foot. The lama explained that while it was fine to tattoo the mantra, she should get it done on another part of the body, preferably above the waist. Similarly, while Thailand has a long tradition of Buddhist tattooing (often performed by monks), the legs and feet are usually reserved for non-religious tattoos.
Of course, if you live in the US or Western Europe, you may not care too much about what Indian or Tibetan Buddhists consider inappropriate. But if you’re tattooing a Buddhist symbol, it seems like you should at least take note of how it would be traditionally used. So before you go and get that beautiful Tibetan mantra tattooed, it might be worth doing a little research into which body parts are, or are not, appropriate.