Really depends on your game. How do you want to mix your ambiences? Is it tile based? Are you laying out areas/volumes?
You certainly can do it either way you describe. Or you could do it all with emitters in the world. You could do a proportional mixing system based on amb types by area. You could do it with quads mixed across the surrounds with accent sounds. I don’t think I’ve approached any games ambiences the same from one to the next.