Is it better to add cream when you first get your hot coffee? Will it cool faster or slower? I found the math answer here at DC Physics. Big formula to say this:
The cream and coffee have a certain total mass which is losing energy to the surrounds, thus lowering the temperature. The temperature of the mixture will be given by the total energy remaining. So, the critical factor is not when they are mixed but how fast energy is being lost to heating the environment in each case. Examine the various contributions (evaporation, black body radiation, conduction, convection)... in each case energy is transfered faster from a hotter object than from a colder object. So, by adding cream/sugar at the beginning the mixture achieves a lower temperature immediately, which means a slower rate of transfering energy to the surrounding environment. Losing energy slower means the average temperature of the combined coffee/cream/sugar will be higher. Simply put you should add your cream and sugar immediately.