Tug-of-war is not a recommended game for dogs because it is a dominance game. If your dog should win you just reinforced in your dog’s mind that he is in the leader position. It puts him in a dominance struggle state of mind. You want to get him out of thinking about being the leader, not play dominance games with him. When he is pulling he is fighting for the leader spot, while for you it is just a game.
Sure, if you win, then you win, but if you accidentally let go then he wins and winning to him means something different than winning to you. Another important factor when playing with a dog is that you should only play with him when YOU bring HIM the toy; you TELL HIM when and where. You should be the one to end the game, not him. So if you see him getting tired then you must end the game before he does. Otherwise you let him make the decision and in the canine world the leaders make all the decisions.