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.

I came across an amusing story on TED about how rigid Japanese can be when it comes to serving food. Sushi could be served this way, but isn't (at least in my experience). Take a first look at the first three minutes of this clip. http://www.youtube.com/watch?v=lDq9-QxvsNU