As someone whom mostly prefers DPS roles, I can say that it's GENERALLY better for the tank to mark targets if you have a first timer in the party. If you are NICE, you can simply explain which ones to kill first in crowds.

--That said if there aren't any first-timers int he party, then it's up to your party as a whole. Some don't like to waste time doing that and will nuke the crap out of crowds and not give a rat's ass while others prefer things be more organized and will use nukes, but like knowing whom to focus on more.

Personally, I focus on whichever one has the most HP and use that as the central target while I nuke and DOT unless tank says otherwise.