Learning it up to 9 is obviously important, unless you don't think being able to do any kind of mental math is useful. But the 10s and 11s are very easy to lea

Learning it up to 9 is obviously important, unless you don't think being able to do any kind of mental math is useful. 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.