i clean my grill a couple of times a season and use the steel brush on it before each use. generally, the heat will kill bacteria if you are concerned about that. I don't like cooking steak on clean stainless steel, i find the build up of charcoal adds flavor (don't let food build up), so i don't ever clean it completely. i've had a lot of good grilled dishes right off the streets stands, and i doubt they clean their grills regularly.
i think though, since you are debating with your buddy, in the united states, most people think it's necessary and recommend cleaning after every use. but we are germaphobes