This might sound a little dumb but I just follow a simple rule of thumb for low level dungeons. If they are a Warrior, feel free to AoE all the time because with Overpower they should be fine to hold hate on multiple enemies. If its a Paladin I tend to be a little more cautious and ask if they can hold an AoE rotation before we start or see if they are marking targets.

In terms of numbers, its really only viable in low level dungeons to AoE at 4 or more enemies otherwise single target is better for maximum DPS. In the endgame you definitely need to AoE a lot more to speed things up.