Have you ever had less expensive sushi, where they put a little piece of plastic leaf next to the fish? That is meant to imitate shiso leaf, which the better sushi places will use on the plate as a palate cleanser. It tastes like a cross between mint and parsley, with a hint of anise.
That is a good fun fact indeed. The place we go to has the best sushi I have tasted in several states, but none of it is offered with shiso leaf. That is why we come here, to learn!