I really can't be sure. I play cod 4 competitively, and running out of ammo in a scrim very rarely happens. In fact I don't think it has ever happened to me. In a public match, running out of ammo happens often, so i change guns almost every 3 kills.
I am also an OCD reloader. Even if i drop an enemy in 2 shots of my AK-47, I will still reload before he hits the ground. I don't think its ever become a problem, as I don't do it if there is a group of enemies.
Also, L4d has taught me to reload before my clip is empty, since reloading an empty clip takes almost twice as long.
Interesting topic!