Aggressivness is one of those "rooster things" that just happens or doens't happen. If you hold them alot and treat the boys like pets, especially if they have hens, then they tend to get a bit snippy. If ou stop holding them alot once they hit manhood, then they can grow up and figure stuff out on their own. In the long run, it all depends on the individual rooster. If you're breeding them, then the temperament of the father can play a rather large roll.
DON'T tease them, make fun of them, or pick fights with them. This will only reassure them that humans are threats.
ETA: Docile breed roo's have been the best rooster's ever for me. Our buff orp is a little "rude" in the morning, and he picks fights sometimes, but he's also gone after dogs as big as border collies, which is a really rare rooster.