I read someone's journal in lang-8.com, he is asking whether "of course" is a polite enough to use because her teacher (which is american) told her "of course" is not so polite because it sounds like the answer is a certainty and no one should not ask such that question.
for example :
Q: Do you like that movie ?
A: Of course, I like it.
come to think of it isn't entirely wrong for me. What about you?