Leveling in dungeons (everything upto Qarn so far), when is it better to focus on a single target and when is it better to AoE, or damage multiple targets (for example put DoTs on all targets)?

My main is Whm, but I'm leveling all DoM. Generally, as either Whm of Thm I prefer to focus on a single target, and sleep the others, but some other players spam AoE and/or DoT everything up.

I understand it's going to depend on group composition, and the trash pack, but;

When does it make sense to AOE trah packs?
Is it significantly better DPS for that class/job?
Is it better DPS for the party (if the healer can't dps because he's healing more is the party dps actually better)?


I really enjoy leveling in dungeons. Obviously I'm leveling slowly, but I intend to continue to just follow the main quest, class/job quests, and use duty finder to level.