I know it seems like a silly question.
I'd figure so, sure, except maybe when the Dogs are in town they might end up doing some stuff, because they're also invested with the authority of the King of Life. I'd guess the Steward still leads services and so on but probably if there's Dogs in town he'll invite one of them to speak too, and maybe if anyone's getting married or has a baby while they're there they might ask one of the Dogs to perform the service rather than the Steward.
But the real answer is: the setting isn't set in stone, so whatever suits the vision you and your group have of how the Faith works, that's how it is.
Ultimately, this is a question that each group answers for itself, but in our group the Steward was the one who speaks the invocation at the beginning of the weekly service, chose the hymns, chose the readings from the BOL, gave the sermon, and closed the service.
If you look in the book, large Towns can have assistant stewards, so concievably they could also perform the same function.