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.
Signal propagation at 2.4 and 5GHz is remarkably tricky, and it's easy to fool yourself about the causes of signal strength differences when you move the AP. So if you're not an RF engineer doing rigorous testing, your conclusions are suspect. Maybe the improvement from moving your AP those few meters had nothing to do with moving them away from power lines, and everything to do with putting it in a better position for multipath signal propagation. – Spiff – 2010-04-01T01:15:59.137