I'm mostly on my BLM or RDM so I tend to have a soft spot for deeps.

However, I try to give the com to people I notice playing their job to it's fullest. (for whatever lvl range the dungeon is.)

Example. If I see another blm who might have stayed too long in an Aoe before moving pop a manaward.

Or players using Mana shift or goad.

Ninjas making agro for tanks and smoke bombing dps who aren't paying attention to to their agro.

Stuff like that.

Though I am guilty of defaulting to the other dps if I'm tired. (Unless they are exceptionally bad)