That recommendation from the seller was true for old batteries, and it seems they still keep saying that to customers!
Nowadays, batteries are often
Lithium-Polymer and such batteries (as I have read many times and based on my own experience) would be stronger if you charge them often. The first time charging and "wait-until-full-discharge-before-recharge" and "don't-use-when-charging" are not applicable to these modern batteries.
Li-Ion and Li-Polymer batteries, if charged often, after about 1 month, would reach to their maximum performance, and you are recommended to charge them every time you find an outlet!
In stock Help application of my Sony Android device, In a section about Battery Life, it reads:
- Charge your phone often, This will not affect the lifespan of the battery.