Originally Posted by Rivet gun
Exeptions are:
You must have QNH set for take off and can't change to STD below flap retraction altitude.
At least one altimeter (e.g. the standby) should remain on QHN below MSA for terrain awareness.
One exception to that:
Our company states that with a low QNH and an early level off (FL40) SPS can be set prior to take off and on both altimeters. This is usually covered in the brief and is at the Captains discretion. The standby, of course, remains on the QNH.