There are two things in backgammon I can't stand: bad losers and bad winners.
I am sick of people complaining about their dice whenever they lose. It's an insult to me ... it suggests that they only lost because I got lucky or they got unlucky.
I am just as sick of people who win, and then proceed to tell you that you should not have slotted the 5 point (which only hurts if they roll the 35-1 shot that they did roll in order to win the match).
When you win, shake your opponent's hand and thank him for a good match, or if you can't say anything nice, don't say anything. Whatever you do, don't gloat.