SOP will vary with each airline and may depend on how low transition is. Airlines which traditionally operate in areas with low transition will usually have the 'altimeter' check on the checklist fairly late in the approach sequence.
One system which seeks to prevent a last minute rush when transition is low, is:
At top of descent set STBY altimeter to landing QNH.
When ATC clear the aircraft to an ALTITUDE, the PF altimeter is set to landing QNH.
At or approaching TRANSITION the PNF altimeter is set, then all three crosschecked.
The idea of this is:
1. If the worst happens and later the crew forget to set QNH on all altimeters, at least PF will have it, and so, hopefully, will the autopilot (installation dependant).
2. If there is a significant split between QNE and QNH, it should become obvious when the altimeter cross check is done on approach that one has been forgotten, whereas if both have inadvertently been left on QNE the error may be missed with potentially serious consequences.
3. Until transition, PNF is able to report any levels required by ATC.
However, where there are crossing restrictions in the STAR, careful monitoring of the vertical path is required, and of course if any intermediate level off at a FLIGHT LEVEL is required, PF can NOT set QNH until ATC assign an altitude referenced to QNH (although there is no reason not to set the STBY at TOD).
In some installations, using such a SOP would require the PNF transponder to be selected for altitude reporting, not the PF transponder.
Not saying this is the best system, but have seen it work well as a SOP.