Using iPhone while charging and when charged
None whatsoever unless you somehow drop the phone since it's attached to a cord.

The charging system on iOS devices is adaptive and only engages when the battery needs charging current. It's smart and slows down the charge appropriately whether you are using the device or not.

Put another way, the device only asks for enough power to run itself and charge if needed from the wall. The wall doesn't send too much power so there's no risk of overcharging.