The presence of three or more mobs makes AoE optimal.
But the vast majority of the time on trash you won't be AoEing, because in heroics all but one or two mobs will be CC'd, and there are a VERY small number of bosses that require AoE in Cata.
Considering this a forum geared towards endgame theorycrafting, I think the question about AoEing makes more sense if you put it in the raid setting. Unlike heroics, the first four easiest raid bosses (Omnitron, Magmaw, Maloriak, and Conclave) all require significant AoE. In most of these cases, you will have more than 3 mobs and need to execute your AoE rotation there.
Magmaw, you will likely be aggroing the adds with DnD and Howling Blast in Blood Presence.
Omnitron, you will likely be using HB and DnD on Toxitron's adds.
Maloriak, you will be using heavy AoE on adds in green phase.
Conclave, you will likely be DnD/HB on adds for Anhash.
Setting up your AoE rotation is just as important as executing it. Have all your death runes converted and up if possible. If you do still have a blood rune up still, DnD> Outbreak (on targeted add) + Pest > HB spam is most likely optimal for adds that have alot of HP (Maloriak, Magmaw). If you have an on-use STR or Mastery trinket, be sure to use it here, as well as keeping up ERW --- unless you are planning on saving it for burn phases or can use it twice throughout the fight.