[Gambit]

Speaking as a parent, (sorry for the lecture)
When someone else is providing for you, you live by their rules. If you don't like the rules, show that you are resourceful enough to either pay your fair share of the bills, or move out. It doesn't have to be hostile, you are just showing you're adult enough to handle the responsibility. (My sister lived with my parents longer than I or my brother did. She ended up paying them rent, and as a result had fewer rules to deal with.)
Then, if you're a "responsible adult" you should be able to make your own choices about tattoos, because you're the one living with the consequenses. A "responsible adult" deals with his/her own problems, including medical insurance or the results of not having it, as well as any social issues caused by one's own decisions (say you had a swastika or something).
I do not recommend sneaking around to do what you want to do in the face of "the rulez." But there are a number of things here that I don't understand:

You asked permission the first time. If you are taking care of yourself, as an "R.A" above, was this necessary? If not, good move.
Having asked for and received permission, why did you ask again?
If you do need permission (parent's signature, etc), how do you plan to take care of it and keep her ignorant?