In the SOPs that I'm familiar with: set STD on main (PFD) altimeters when cleared to a FL and above acceleration altitude. The standby gets set to standard once above TA and MSA.

 

On the way down: set the standby to airfield QNH before TOD as part of the descent preparation, then set airfield QNH on the primaries once cleared to an altitude.

 

Thanks simon. So what is the benefit for setting the STBY alt to the SPS above MSA and QNH during the descent brief?

I couldn't tell you for certain, but thinking logically I would imagine it ensures that you've always got a QNH reference *somewhere* in the flight deck whilst you're below transition (and MSA). Likewise, setting it to destination QNH during the approach brief, at a relatively low-workload stage of flight, means it's less likely to be forgotten later on.