I would set standard when I have been cleared to a FL and once above the MSA most of the time, however, Some SID's have not below altitudes for terrain clearance even though the final level is a FL. So for navigation purposes I would want QNH set for longer. If the final level on a SID is a low FL in an area where the transition altitude is low, you may want to set Standard soon after take off especially when there is very low pressure as setting it late could result in an alt bust. I don't believe there is a single rule that will cover all situations, we need to use common sense and airmanship as each departure is different. In the UK when cleared to an altitude in the descent we tend to set QNH straight away, this never seems to be a problem. When in Europe the controllers seem to expect you to set QNH when passing transition level and not before. Out of interest what do you call standard once you have set standard? "Standard" or "one zero one three" or "ten thirteen"