Learning it up to 9 is obviously important, unless you don't think being able to do any kind of mental math is useful.<p>But the 10s and 11s are very easy to learn, so there's not much reason to stop before 11. I suppose there isn't much objective reason to include the 12s. They make a nicer endpoint than the 11s, and are no easier or harder than (say) the 6s or 8s, but they don't come in handy nearly as often.