LiPo batteries, as used in most all portable gadgets nowadays, do not fare well on the long run with deep discharges. Get the battery level to show up onscreen so you can keep tabs on it, get a feel of how fast it drains given your usage habits, and recharge when you feel it won't get you thru the next day. Say, when at less than 30% or so?
Should I let my phone completely drain before recharging ...
... can I charge each evening?
If you like.
More Info here > http://www.apple.com/batteries/iphone.html