I recently installed iOS 7 on an iPhone 4S. Does the Battery Usage/Percentage (as shown in the top-right corner of the screen of the phone) need to be calibrated again (by letting the battery fully discharge once, possibly to be followed by a full charge and a hard reboot)? Or is this nonsensical advice?
Discharging the battery once in a while (every few Months) is not a must but does help calibrating, for the system to be more accurate waring you on low levels.
Over time charging and discharging creates a more inaccuracy of the actual level. For example if you plug in at 40%, unplug at 80% and so on.
The reason is that the system uses the last known battery charge to produce the %, thus assumes the battery charge level.
However if the battery was not discharged for long time, the remaining charge could be wrong (due to batter aging).
Discharging to not less than 20% once in a while helps the accuracy.
Since there is a opposite effect (to frequent full discharge) will shorten the battery life, you should only do this every few months.