Wasabi should be put on sushi before touching the soy sauce for premium sushi, when fish are very fresh, and ingredient are excellent. First, in good sushi place, the wasabi is already added to sushi by the chef, a dip of sauce is enough. (Most of the time soy sauce is too already added.) Secondly, if the wasabi is freshly ground, the taste will get lost with the sauce. Thirdly, the wasabi soy sauce tends to overwhelm the delicate taste of the fish and seafood. It’s a bit like with a plate, one adds salt and pepper without considering and tasting the seasoning done by the chef.
I do mix wasabi (from a tube) into soy sauce when fish is of mediocre quality, and when that extra seasoning is needed. (I used to do this a lot because I like soy sauce! But it’s a waste on good sushi and I can eat this with plain rice!)
