We had quite aloud debate today at work about whether you should charge brand new Li-Ion batteries for 16 hours or not. I was sure it's not necessary but I had to prove my stance.
I did some googling and now I'm absolutely positively 100% sure you DON'T need to charge new Li-Ion batteries for 16 hours. This is one of the stories that tries to go deep on the subject and don't just say "do it just to be sure". Of course, there are people that say just that, but they all say it from the top of their head, no one has given any proof or link.
So, any warning that says you should overcharge must be general and it's not specific to Li-Ion batteries. It covers all kinds batteries.
The reason why overcharging is good is obviously this thing called "trickle charge" and it exists in Ni-Mh batteries. This is a charging phase that follows the "fast charge" phase (that's when the phone/camera/other device shows that the battery is full). Li-Ion batteries and charges do not have "trickle charge" and when your charger/gadget says battery is full, it's full.
Another thing is that Li-Ion batteries do not have the memory effect. So you don't need to fully discharge and then fully charge your batteries every time.
But yes, your battery will die on you. Soon or later.
Probably the most useful information I found is a table of do's and dont's for different kind of batteries. I think it tells more or less everything you should know about your batteries. Check it out!
Your sales person probably told you that it's really good to over charge you batteries for the first time. Best reply to this statement was given by a guy named James at alt.cellular.nokia newsgroup: Sales clerks are not allowed to know what they're talking about.
Some links if you would like to check things for yourself: