Generally, you don’t add wasabi to sushi - nigiri, maki, or gunkanmaki (the “battleship roll” like how uni or ikura are often served). There is already wasabi between the rice and topping / filling.
You add soy to sushi that isn’t already garnished with a sauce (EDIT: e.g. a soy-like sauce, yuzu and salt, yuzukosho, even some marinated grilled items, etc.). You dip it fish-side down - try not to get the soy on the rice. You can get a corner and slightly roll it to do a light coat of soy. That’s why eating sushi with hands is preferred, because you almost always eat it fish-side down, anyway, and such a maneuver can be difficult with chopsticks. If it has very delicate garnishes on top, that would fall over if turned, or if it’s cooked - e.g. a broiled piece of saltwater eel - you can eat it with rice touching the tongue first. Generally, you’d rather err on the side of caution with a less soy rather than excess soy.
At many places, however, the sushi may come sauced already by the sushi chef.
If you want to add soy to something like a gunkanmaki, don’t dip the rice. Rather, the better way is to take something like a piece of ginger with your chopsticks, brush it with soy, then brush the topping with it. This really only applies to undressed uni or handrolls, however. Ikura usually has a marinade, meaning no extra soy is needed.
You do add wasabi and soy to sashimi, however. Note: real wasabi tends to be milder than the fake wasabi. So, you can use a little more if it’s real wasabi, especially if it’s Japanese, e.g. Shizuoka. Fresh wasabi from California is “hotter” and less sweet than Japanese fresh wasabi, typically.
It’s somewhat personal preference, but some practices will ruin the balance of the sushi or drown out the best flavors (e.g. dipping the rice, adding extra wasabi to sushi, etc.)