I voted yes, but I didn't take teaching to be in any formal capacity out in front of a specific class. I'm ranked 3rd kyu with three and half years of training, I help teach in the kid's classes and help out the newbies by teaching things like hanmi and rolls. I really have found that in teaching I discover how much aikido I know or don't know for that matter. I'm barely an advanced practitioner but I don't see how one can avoid seeing that helping a less skilled partner improves your own skills.
As to the "necessary" part, yes it is. I see teaching and learning as opposite sides of the same coin. You can't have one without the other. In order to learn more deeply you need to know how to teach, and to teach more effectively you need to know how to learn.