from what i've learned from italians and italian americans, it's a fallacy that cheese should never be added to a fish dish. just not usually.
it all depends on the dish, as with any good cuisine. for the most part, subtly flavored fish would be overcome by the stronger, salty cheese flavors, so it's a no-no. italian food is all about a few, perfect ingredients in harmony. but there are stronger fishes and those with appropriate sauces that are good with cheese, as they can handle it.
this myth has been repeated so many times that even most italians believe it and automatically refuse cheese in public, but will whisper that they often add cheese at home.
think of it as automatically adding a load of salt to a dish before tasting it. if you add cheese to something in public, you're suggesting that the fish is not fresh or tasty in some way, and needs to be masked.
but really, it's all about the food, and the eating experience. so if it tastes good to you, grate away.