I always try and do this either in-game or in the post game lobby. Win or lose
But (and I don't know if it is a low elo thing or not) I don't see many people do this. More often than not there is hostility.
It is a universal act in any sport or game that the players all shake hands and say a few words of congratulations at the end, but it is something that seems lacking in LoL.
I have even been flamed for doing it. It seems wrong not to be gracious in defeat.
Do you do this? If not, what is your reasoning?