I don't know how the army does it now, what with different rules of engagement but when I was in Vietnam everyone in the squad carried their rifles with a round in the chamber and safety on except for the point man, he had his safety off and selector switch on full auto.
Fast foreword a few years. I did the rifle training for our (Anchorage Police Dept) SWAT, (we called them CRT). I taught leave the rifle on safe until you're ready to fire.
It only takes an instant to flip off the safety. I can't think of a situation where the time it takes to flip the safety would make a difference. Especially on a M16/AR style rifle.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071