Lithium batteries like the type in your iPhone are different from nickel batteries. Phones that use nickel batteries are particular about how the phone must be charged, and often the phone must be fully discharged to preserve battery life. Nickel battery phones are also prone to overcharging. With a lithium battery in your iPhone, you don't need to worry about when to plug your phone in to charge, and you can charge the phone at any point without damaging the battery capacity.
The iOS charging circuits will actually stop the charging process and let the battery discharge a little bit from time to time. Apparently this helps in battery life.
PS: For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Be sure to go through at least one full charge cycle per month (charging the battery to 100% and then completely running it down).
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