Soy sauce on Sushi is a total Faux Pas in Japan; yes, Soy sauce does come in the boxes of Sushi on sale in the west, but try it on in Japan and you will get laughed at, its like putting Ketchup on boiled veg to them. Not only is it just not on, it totally clouds the true flavour of the sushi too, Sushi is supposed to be delicate, Soy Sauce is very heavy, it totally overpowers the natural flavour, and is full of salt. Salt is bad. Soy Sauce is bad.
As far as being healthy goes, there are much worse things you could eat!