First time caller, long time listener: I've seen this question asked on a few other sites but with no "definitive" answers.
The Apple website on iPhone batteries is ambiguous, at best, and the closest it comes to answering the question is saying that a monthly deep cycle is a good idea. (reference)
So, what do you think — is there an adverse impact from repeated, overnight charging of your iPhone 4? Does this negatively affect battery performance over the long term? Or is there some chip/ software that acts to shut off the charging once the battery is topped off, thereby protecting your hermetically sealed little power source?
Best Answer
All this hype about how to charge your batteries is blown out of proportion.
Although some batteries (I forget which types exactly) can have longer lives if charged at "optimal" rates and temperatures, the reality seems to be that it does not extend the life by much. One of the research papers I read concluded something like if you charge your device at around 17 degrees from 0% until 75% full you can extend the lifespan by about 1/5 of the total life span.
I have charged my devices for days at a time, and my iPod Nano has been plugged in for months, and holds a charge the same as my other nano (I have two) which almost never gets used and only charged ocasionally.
In conclusion, just do whatever is convenient for you.
EDIT: Here are some links to the apple pages related to their iPhone batteries: