You should definitely tell her! I'd feel sad if I knew that my boyfriend had deliberately kept it from me, especially if he had told loads of strangers on an internet message board! Yes, it's your body, and you can do what you like with it; your girlfriend probably agrees with this. Here are the scenarios as I see them:
Your girlfriend hates nipple piercings.
*if you tell her beforehand* - she can ask you not to, and you can explain that you want it done but that you wanted to talk to her about it first - she will hopefully be glad that you value her opinion and didn't do it behind her back, and it wont be such a shock as being presented with the piercings themselves.
*if you do it secretly* then it'll be a shock when you just show them to her, she'll feel sad that you didn't even talk about it to her beforehand.
Your girlfriend doesn't mind.
*if you tell her beforehand* she'll be glad you spoke to her about it first, might even come with you to hold your hand!
*do it secretly* she'll wonder why you didn't even mention it first
Well, this is how I would react in the circumstances anyway. Not telling her just seems to be a losing situation, however she feels about nipple piercings.