If the developer is a replenisher and the literature says to use the starter, then I would use it. All C-41 developer/replenishers need starter to supply necessary ingredients to make a proper working developer, whether used in a machine or by hand. That is why it is called "starter". (The ingredients are not needed for replenishment.) Otherwise, you will not get optimum, in-spec results. If you do not use it, then be prepared to accept whatever you get.
David, IIRC, you process at low temperatures with dilutions so your results are not optimum in any case.