Someone told me I should shut it off every so often, because it helps everything reset.
iPhone 3GS
And you don't actually want to run the battery down to 0%. You just want to run it until the device shuts off on it's own. This calibrates the battery sensor. Not ever doing this won't hurt the battery; you just may not get accurate readings on remaining power. It's not good to drain lithium ion batteries completely.
Really, never heard that before. I've got a two year old 3GS that has NEVER been turned off. Runs/works fine. If it ain't broke, quit trying to fix it. Turning your phone off, then back on is a basic troubleshooting step. So, having trouble with your phone?
I do not shut off my iPhone 4, my iPhone 3Gs nor my macbook pro (nor my nook ereader for that matter) - they sleep when idle but that is it. They only even get restarted if a software update or something requires it. Otherwise, they are on 24/7 and have been for years (well some of them, the iPhone 4 is not a year old yet).
Lithium batteries do best with regular use and partial cycles.
I never do. If it seems like it's bogging down, I'll shut it down and turn it back on or reset it (hold home & power till you see the Apple logo).
Where did you hear that from? Its good to completely run down the battery once in a while, but not to shut it off.
How often and for how long should I shut off my Iphone.
