I think just about any level of fluff is fine so long as the player doesn't try to leverage it for mechanical benefit. As a GM, if i sit down with such a player, at least I know he's familiar with the world or has read up on it, or possible has some awareness of story lines, all of which implies that he's that finest of all players: one who pays attention.
Without divulging spoilers (directly), I have a character who's a member of a certain far-northern aristocratic family - a member of which was (possibly) spared in a Year Zero scenario, thus granting the favor of that family. My logic was that one of my high-level characters had that year Zero boon, and when I retired the character, it happened that she took on a young ward - a member of said aristocratic family - and said ward is now a PC. So it's a background which played logically from my characters' lives, even as it's bound to "in game" specifics.