The optimal setpoint is seen as between 23°C and 24°C – both during summer and winter. This is enforced by Health & Safety regulations in the workplace. However, this will differ in certain situations : for instance server rooms and other equipment rooms would need to run at a lower temperature.
For domestic (household) usage, the temperature can be set for your own requirements. Remember that the unit can only perform according to its capacity. Therefore, setting a unit to a lower temperature (i.e. 18°C) will not force the unit to cool the room down any quicker than setting it at 22°C. The unit will still cool at the same rate. Setting it at 18°C will eventually get the room cooled down to 18°C if the unit is correctly sized. Installing an inverter unit is the best way to achieve the desired temperature.