Most commonly, sushi is eaten with minute amounts of soy sauce. There are people who add soy sauce to the topping and those who add it to the rice. Still, the former method is considered correct.
However, with some toppings such as gunkan-maki (the battleship roll), turning the piece upside down would make the topping fall off, so the soy sauce is applied from the side, or just a little is dabbed onto the rice. Connoisseurs apparently dip pickled ginger in soy sauce or spread the sauce over the seafood.
Some toppings, such as conger eel, are not eaten with soy sauce; instead they come coated with a sauce called tsume, made by simmering down soy sauce. In that case, no extra soy sauce is added.
On a related note, bear in mind that in high-class establishments, the chef may spread the soy sauce himself and offer the sushi like that.
