We teach our student to fire that first shot in DA if they are carrying a DA weapon. It will depend on your situation but for the most part it should be fired in DA.
For civilians, in most gunfights, you are the respondent of an attack and already playing catch up...and seconds count in a gunfight. Under stress in a life-threatening situation, when "fight or flight" has kicked in and fine motor skills are no longer present your best bet is to get the weapon out of the holster and on target as quickly as possible to engage your threat. Other than clicking the weapon off safe there should be no additional steps required before you fire.
It's vital to practice your draw in both live and dry training sessions to build good muscle memory. Good repetitive training will ensure that you react instinctively with the mechanics of shooting, leaving you more thought process for judgment of your situation.