You should definately, surely know proper technique in posture, right hand, and left hand. Theory-wise, I agree, you just need to know something more than the student. If you want to become a professional teacher, however, you need to know about different styles, know your theory inside out, and be prepared to answer a hoard of questions. Having internet access is helpful during lessons if a student requests you play a song they've been working on or something they want to learn, so that you can quickly look up the tab.