IMHO, I say let her pierce her navel. Heck, take her yourself so you can ask all of the parenting type questions--you know, the one's she'll forget. THEN nag her to make sure she takes care of it the right way!

One of 4 things will happen:
1. She will think you are the coolest and brag to all her friends and it will bring you closer together (no better way to bond with teenager than to be a cool but responsible parent).
2. She'll be completely embarrassed that you are taking her and chicken out.
3. She'll get it, agree with item #1 and become more self confident in who she is because you respected her decision for her own body
4. See #1 plus, eventually get annoyed at you for being the responsible parent by nagging her to constantly keep it clean etc and take the darn thing out!

Either way you go, the job is to be the parent first and help her learn her own path in life. GOOD LUCK, I'm not a teenage but was one a few short years ago. And I can tell you, a little personal choice freedom goes a long way to preventing the major rebellion!