It's interesting to see the competition between how large a gmail account you can have.
I delete all I can, trying to keep my mailbox(es) as trim as possible. About once a month I take time to delete everything I possibly can, keeping only items which still need a response somehow. My inbox currently has 53 items in it, my archive maybe 300 total, and that's only because I haven't had time to trim it more.
I think it's because I learned email on a VMS/VAX with limited quotas. It also helps to reduce evidence that I'm completely barmy, which email seems to reveal more than other writing.
-Jared