I'm a little surprised that the question presumes that there is a correct way of eating sushi.
Personally, I think that if there is a correct amount of wasabi that should be applied to sushi, the sushi chef would not offer a little extra on the side. Food is to be enjoyed. Taste is subjective. If you like ketchup on your sushi instead of wasabi, then enjoy your ketchup-laden sushi (and ignore the others gasping in horror).
It seems to me if sushi rules were so stringent and there was a right or wrong way, options like soy sauce and extra wasabi would be removed from the equation.
Garrick Saito, likes food
Wasabi should be added to 'perfection' by the itamae (sushi chef), so you shouldn't need to add any extra wasabi yourself. (Doing this might be frowned upon by the itamae, like salting the food before tasting it.) Although this is often customary in Japan, in Europe extra wasabi is often served on the side for customers to choose the amount themselves. Normally people mix it with the soy to taste.
My experience with sushi in Tokyo is that nigirizushi is not served with extra wasabi, but some makizushi is. For example, kappamaki (cucumber roll) is often served with extra wasabi on the side. At places serving kaitenzushi (conveyor-belt sushi) extra wasabi is sometimes available at the counter, like gari and soy.
Having said this, I myself add a little bit of wasabi (or wasabi substitute) to the soy. At least when eating sushi outside Japan. As with all food, you should use condiments to your personal taste.
Johanna Forsberg, Japanophile & photo enthusiast
The tendency among Japanese people I've observed, both in the USA and in Japan, seems to be to use it more sparingly than most Westerners seem to, and often to not use it at all. Same with soy sauce, for that matter.
But it seems to be highly subject to personal taste: I've seen Japanese people (in Japan) douse sushi in so much wasabi as to render the fish undetectable, and I've seen a high-end sushi chef completely remove the little plate of wasabi from the sushi bar when he discovered that his customer was Japanese.
Save enough wasabi so that you can load a ridiculous portion on to your last piece of sushi and eat it to impress all your friends.
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