After a number of tours in the box with different units I can say that every unit is a little different. I imagine most police departments are a little different too.
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.
Not always. I have seen it done that way but most training for CQB that I have done involves the weapon remaining in firing position during most room clearing.
During dismounted patrol the standard is safety on with it coming off when raised to the firing position.