We had a native Japanese poster in another forum offer the following option of applying soy to your sushi.

Take a long piece of gari (ginger) with your chopstick, dip it in the soy (or soy/wasabi mixture if you're so inclined), then "paint" the mixture on the upper surface of your sushi fish so you apply just enough for flavor without soaking the fish or the rice.

It also keeps the mixture clean of dropped rice and fish oils.