Typically it depends on the game, my weapon, and other factors. When I'm sniping in TF2 I go for the headshot - it's the most efficient way to use the bullet. When I'm playing say BC2, or a singleplayer FPS, I usually go for a few headshots to thin mobs down, then I go for bodyshots with an SMG - because you're much more likely to hit the chest, with an SMG you can do it with many bullets in quick succession, so it normally puts the target down as fast as a headshot, with a much higher chance of success. Ultimately, I'm more about taking my enemy out as efficiently and as quickly as possible with the highest chance of success than about doing it in a specific way.