Oddly enough three of the more advanced sources don't even mention it; ColdBore, Field Firing Solutions and Sisk's articles.
All of them imply other sources of error such as incorrect click value and range uncertainty are more important.
The one print mention I can find is from AB and they say to measure it at the turrets. Given their emphasis and video on the subject of click value it makes sense, but I can also see where an aggressive scope cant might matter here.
Seems even Leupold agrees on turret not objective: