Yeah, I don't think this has ever been said to me in a serious manner.
If it were ever said, like with many things I haven't planned for, I wouldn't know quite how to respond.
Personally I think the concept of a person being someone's hero, is not something that should be perpetuated. I have people that I admire for certain aspects of themselves, but I've never seen anyone as being my hero. I wouldn't want that title placed upon me. That being said, I think the expression "you're my hero" in instances where that person has just saved your life (or something along those lines) is okay, because it only references that one action. It's not tied to the person as a whole, thus doesn't place an unending burden on them.
^ Wow, I really went off topic.... . Okay, maybe just pay attention to the first part for an answer to your question.