i usually play on my games until the very end .. although i sometimes already know there wont be a way for me to win anymore

the reason for that is that i like to get a feeling with the end stage of the games .. for example in backgammon i play it on till the end to see wether a gammon or backgammon would be won or what the consequences of the positions would .. and how my opponent handles the end stage

the same for chess games .. i am not a good chess player .. therefore i play it on to see how and end game can be played

in games like dice poker i resign when i am too many points behind .. the same for reversi where i can count out the number of pieces which we will both have in the end

though when i notice that my opponent can reach an achievement then i wont resign and play on
(for example the achievements in dice poker can only be obtained when you win the game and not when your opponent resigns ... so when my opponent has all the bonusses i do play it on till the last move)

i would consider it a bad manner if i would resign such a game .. if i really want to resign i send a message to my opponent if i can resign on the next move ..1 more move wont matter

its good manner and good etiquette do place yourself in your opponent position and think of what (s)he might prefer you to do .. then weigh it against your own wishes .. and make a good decision .. or discuss it with your opponent