i told mine from the start, as i see no point in a relationship if you have to lie and hide about something like this. I am who i am and do what i do and it was up to her to accept it or not. If she had found out sometime later, or just found needles about the place, she would have just assumed the worst and it would have been worse again because i'd have been lying to her.
I explained everything to her, gave her some stuff to read and i told her why i do it, and she is quite happy with it.
Won't work on all women though!!