It really depends on what the party is doing. If targets are being slept then you need to switch to a single-target rotation. If the other DPS is focusing then you need to focus as well. If the tank can't hold aggro well and you find that doing AOE is peeling mobs away from the tank then you need single target.

Conversely, if the other DPS is doing AOE's, and the tank is doing a good job of building threat, then you should be doing AOE's as well. Both strategies are valid and the performance of one or the other strategy depends on party makeup.