Piercings are no good for animals. When they itch they might rip it out. Or when they roll, play, ect... They have no idea that they have to be careful around this thing somebody stuck through them. It not a matter of pain when the peircing is done, its a matter of how easily they can damage themselves when they rip it out.

Tattoos I'm ok with for id purposes and to show information about them being altered. I live in an area where most places still don't check for microchips, so a visible option is great.