Depends on your ability level I think and your personal goal. Mostly those you see are looking to do a personal best, compete in the event or some such as that. They will run to warm up and by the time the event starts their muscles are ready to take on the burst of speed that these 5k runners demand of themselves.
Me, I warm up as I go and enjoy the participation of the event. I feel that I would be too tired too; I am not fast enough for this warm up to make that big of a difference in my performance...