I think it's good the background plays a role. A Stannis-Lannister alliance e.g. would feel ridiculous. It's not metagamey to use the character background from the book for decisions; actually it's rather the opposite. Nobody is forced to do this but I think it's great if you have the feeling to recognize a character. We don't have just some meaningless factions without any history with each other. You don't need to read the booky by the way, all relevant informations can be easily found via google.