I have all my e-mail from the day I purchased my storm (3 months ago) still on my storm. My question is, is it bad to keep storing my e-mail on my phone? Will this cut into my application memory?
Personally I delete the emails on my phone about a day or two after I receive them. I think it keeps my phone running smooth and doesn't take memory. I have all my emails on my computer email accounts anyways so if I need to find one I can just go to the computer. And with my bb email I only use that to get app installs so that's not big deal either.
You can always configure your email server to leave the messages on it. I'm sure you'd reach a full point one day, but that would let you delete stuff from the Storm (or better, set the option to keep them for maybe 30 days).
If you use Outlook, you can set it up to keep and archive as needed.
If you feel a need to keep them, then do. Once I delete mine I always have a copy of them on the server at work. Yahoo is the only e-mail that I found that always me to delete them both from the BB and my in-box.