I believe its when the game ended.
Not sure I'd be bothered in delaying the end though for a few extra points on my ratings. In fact it would be impossible to choose the right moment without knowing exactly what's going on with your opponents other games!
I for one, am not going to worry about it! :0)
are they being calculated to the current rating of the players when the game ends or when it had started? If the latter is the case, does it make sense for players to look for the best moment to end the game?