Try to bring it up tactfully I say. My parents had no problem with me doing it when I actually lived at home (I'm 25 ftr) as long as it wasn't in the house or around them (they didn't like the smell). But, if I wans't sure if my parents had a problem with it, just watch some montell williams when your parents are around, wait til he starts advocating the Sacred Plant for his MS, and try to innocently make discussion about it. See what their feelings are about it (they'll probably ask if you've tried it, I was straight with my parents when they asked, you may say something else if you feel you need to) and then act accordingly.