How important is the first initial battery charging session on your new cellphone?

I have heard from cell phone dealers that when you first get a cell phone, you are suppose to charge it for hours (say 6 hours for example), even beyond when it says its fully charged on the first initial charge.

Is this still true or just some practice that may pertain to older phones/batteries. Even though some cell phone manuals say to charge for an extended period initially, and some don’t (I just got a new phone which did not say how long to charge it for initially).

So I guess, what I am really asking is what is the best practice when I get my brand new cell phone to ensure I get the maximum battery potential from it?