To add to the debate, and this is purely my own personal opinion, below climb MSA, the standby should always be set to QNH. Whether you extend that to "below TA" is a matter of personal choice (or SOPs where dictated).
If you are cleared to a FL, the altimeter that is slaved to the AP/FDS or AFCS or whatever you call it should ALWAYS be on standard. If anything distracts you, as LEM points out, that distraction should, at least, not be the cause of a level bust because you forgot to change from QNH. Whether it is the standby or the PNF's altimete you leave on QNH is a matter of debate.