Re: Becoming an instructor
There's a difference between being an instructor and starting your own dojo.
The best rank to start teaching is when your teacher says you should. In my case, I started subbing occasionally at 1st kyu, and teaching regularly at shodan.
The best rank to open your own dojo is when you have no choice: there are no dojos in the area at all, or none where you feel able to learn. The lower your rank, the further you should be willing to go to avoid taking this step.
The big risk of opening your own dojo, no matter what your rank, is that you stop learning at that point. If you're going to keep growing, you have to be willing to be wrong. That's harder when you're in charge than when you're just another student.