I'm a little late to the punch here, but boxing21 is absolutely correct. It is part of the action of bringing the rifle to firing position in CQB. That's not a book answer, anybody who has trained and conducted entering and clearing a room in a regular Army unit does it that way. There may be exceptions, but that's the standard. It becomes muscle memory, to the point that I still flip the safety off my Armalite when I bring it into firing position, and I've been a civilian since 2009. Entering where there is a known or even a probable threat, your adrenaline is high enough that you are relying as much on that muscle memory as you are on thinking through your every action. That is why all the training, and training, and training. As was mentioned before, any time at the low ready you are back on safe, even if it's long enough to stack on the next door.
Muzzle awareness is of utmost importance in a stack.. I always taught my section to "buttonhook" the first man into a room, it takes a split second longer, but the 2 man doesn't flag the 1 man that way.