Disclaimer: I don't really know much about how to interact with Korean idols since I don't really pay attention to them too much... I always try to be very respectful whenever I'm meeting a new person regardless of social status, age, etc. I have called a homeless man "sir" just as I call one of my father's friends "sir." I would just stick with being polite and use honorific language. I'm guessing that if the idol is comfortable with fans using informal/casual speech, and the fans use polite speech, he will tell them it's all right to be informal. On the other hand, if fans use casual/informal speech, the idol will most likely not be offended since he understands you come from a different culture. Just my thoughts. =)