Also, because I just answered this in another, almost similar question:

The point is, like with Naval Officers wearing an uniform which looks like this and includes the full uniform, the hat, the rank bar and the code cylinders used to enter restricted space / stuff:

The Stormtroopers are wearing their own uniform which includes Helmet and a complete set of armour and are not permitted to remove any of this. I admit, at some points, you see Officers (like at the meeting in EPIV) not wearing their hat, but I assume thats not common when on duty.

Also, to undermine my "Its protocol to wear the entire uniform on duty" statement, heres a quote from EPVII:

CAPTAIN PHASMA: FN-2187. Submit your blaster for inspection.

FN-2187: Yes, Captain.

CAPTAIN PHASMA: And who gave you permission to remove that helmet?

FN-2187: I'm sorry, Captain.

CAPTAIN PHASMA: Report to my division at once.

So taking off the helmet - no, because of protocol.

Also, because I just answered this in another, almost similar question:

The point is, like with Naval Officers wearing an uniform which looks like this and includes the full uniform, the hat, the rank bar and the code cylinders used to enter restricted space / stuff:

The Stormtroopers are wearing their own uniform which includes a helmet and complete set of armour and are not permitted to remove any of this. I admit, at some points, you see Officers (like at the meeting in EPIV) not wearing their hat, but I assume that's not common when on duty.

Also, to underline my "it is protocol to wear the entire uniform on duty" statement, here is a quote from EPVII:

CAPTAIN PHASMA: FN-2187. Submit your blaster for inspection.

FN-2187: Yes, Captain.

CAPTAIN PHASMA: And who gave you permission to remove that helmet?

FN-2187: I'm sorry, Captain.

CAPTAIN PHASMA: Report to my division at once.

So taking off the helmet is not allowed, because of protocol.