Thad: I am not experimented enough to say it for sure, but I think that the recognized backgammon etiquette is to carry the game on as long as there is a theoretical chance of winning (even if it is a one to a billion shot), and resign as soon as there is no more. Usually one assumes proper play by the opponent when computing theoretical chances.

But as it is often longer to compute whether a theoretical chance exists than to play on, in live play one often simply carries the game to the end. In turn-based play, I think it is nicer to bother with the simple computing and resign on time. Note that the Dailygammon server implements that etiquette by resigning the game automatically when a win is impossible.

In matches one should normally be allowed to resign the type of game one decides to (normal game, gammon, backgammon) and the opponent should be allowed to reject one's resign offer if he finds in not high enough. This is not possible on BK though.