if you go to a quality sushi place, the wasabi should alraedy be put directly onto your fish if you are eating nigiri. you simply dip your nigiri in soy sauce, using the fish itself to touch the liquid in order to prevent the chance of your rice ball breaking apart.
that being said, most people in the western world are not eating sushi at high quality places. IMO, its perfectly fine to mix wasabi into your shoyu and then dip as said before in this case. another reason why this matters much less is because most peopel in the western world have never even tasted/seen/smelled fresh wasabi. if you go to a sushi place and are payingn less than 100$ for your meal, you're wasabi is likely coming out of a tube. this stuff is much cheaper than fresh wasabi. Wasting the fresh stuff by mixing it in soy sauce and leaving a lot of it unused would be a waste of an expensive ingredient, whereas wasting some green wasabi paste out of a tube is not as big a deal.
personally i have never even seen fresh wasabi before and ive been eating sushi for as long as i can remember. hipsters will tell you you must follow these rules... and then go on to eat a fuckin california roll from some shithole sushi place in a landlocked state