From what I remember from the previous iPhone, it asked for a pac when you activiated it in iTunes. I already had it so I don't remember if there were options to wait. I've got a feeling that if you want to keep your number this has to be sorted out from the start, it's not something you can do afterwards. But don't quote me. Someone on here will know better.
Anyway, as the pac is valid for 30 days I'd go ahead and get it. Nothing happens if you don't use it.