Shari, interesting approach. We did it differently: we decided DS needed the skill of showing his work (he's a mental math person too).

Once we taught him how to do it and why it was important, he didn't mind doing it any more; I think in his case it was really that he couldn't see why anyone would need to see "shown" what sprang to his mind instantly. But it's a good skill for later, both because computation does get more complicated and harder to hold in your head, and because for things like geometry you can't even pass without showing all the steps in a proof.

DeeDee