4 is the magic number in any AoE situation. If the count is 4+ then using your AoE skills is recommended. 3 or less its better to cycle through enemies.

There is however one exception to this rule, Bard. Bard should always cycle through the enemies and apply their DoTs while using their songs and filler skills. Instead the enemy count is use to determine the songs to be played first. Less enemies Wanderer's Minuet, more enemies Mages Ballad. And use Army's Paean, as needed/filler.