I disagree. Yamada Sensei has been quoted that you grow faster when teaching other students. I've been teaching twice a week for over two years now, and I think I've advanced more because of it. Students ask questions that make you think about how you are moving. And the ability to explain it to others also reenforces your own understanding. However, There are two pitfalls. One, you can be to become attached to the teaching where you stop listening to students and stop training for yourself. (the ego comes in) The other pitfall every time you teach you take away a chance to train (which for me, effects your stamina).
I absolutely agree that teaching helps you learn. But, as I mentioned, teaching within an existing dojo is very different from opening your own place.