Because 802.11n uses MIMO and MIMO thrives on multipath, 802.11n APs don't use directional antennas. So the orientation in which you mount it makes little difference, but where you mount it can make plenty of difference.
Think about where you'd want to stand in your house if you wanted to announce "dinnertime!" once without shouting too loud, and still have everyone in every corner of the house hear you. That's probably also a good place for your AP. Although RF signals don't propagate quite the same way sound waves do, this is still a good rule of thumb for initial AP placement.