For playing through the game, I always keep my Starter on my party. It just feels more comfortable to do so and in the case of Fire types, the selection of them is rather limited, when it comes to Diamond and Pearl. But even with the two other types that have a larger selection, I always remain with my Starter throughout. Exceptions, of course, stand when I rarely pick up a Playthrough Challenge. For one, the only Challenge I've ever participated in was back in September/October. XD;