IMO... I would say you should generally be focusing on single target for trash mobs if you are in any dungeon pre-50, with a few exceptions. Typically if you get a really really large pull they tend to have lower hp and are more viable for AoE.

Post-50, you will eventually be able to outgear dungeons much more so than you can in lower level stuff, so it's definitely in your best interest to AoE in these situations when your tank is geared well enough to hold as many enemies as possible.