Unless it's a challenge run, I always use my starter. I grow too attached to them to put them in the PC, at least for a while. In the most recent games, I actually did put my starter in the PC for a bit because it was getting way too overleveled from Exp Share as I tried to train up other Pokemon, but aside from that, my starter is always an integral part of my team. Actually, back in Red, if my starter wasn't in the first slot in my party, it had to be in the second slot, or else I was concerned it would be unhappy (even though happiness wasn't actually a mechanic back then, and when it became a mechanic, it had nothing to do with party slots). I would go through multiple party-juggling steps to move my Blastoise from the first to second slot and then move whatever Pokemon I wanted to use to the front, rather than just switching Blastoise with the target Pokemon directly.