The rice is already seasoned with sugar and salt (quite a lot of it, actually). Also, the flavour of the rice is enhanced dramatically by the air between the rice kernels. Part of the art of nigiri sushi is making sure that you squeeze it enough that it sticks together, but not enough that the air is forced out. If the fish has not been seasoned, it's usual to dip it in soy sauce (by turning the sushi over and just dipping one end in it), but frequently the fish (especially edomae style) is already seasoned, so you need very little soy sauce if any at all. The sushi should already have the correct amount of wasabi on it and frequently the person serving you will ask you how much you like wasabi so that they can judge the correct amount.

The rice is seasoned using rice vinegar, sugar, and salt; and done to compliment fish or whatever is draped over it. Dunking it into a bowl of soy/wasabi negates the flavors of both. Typically if wasabi is served, it's done as a tiny smear between fish and rice, and the top of the fish will be lightly painted with what ever soy-based sauce the chef chooses. This way the flavor of the rice and it's seasonings is preserved.