When the engine starts, that's when the costs are exhumed, and not on the Hobbs when the master switch is turned on.
Actually the major costs are exhumed from wheels off to wheels on, this is what counts for engine hours and the fuel used while taxying is relatively low, so if working on cost that is how you should charge - but it seems to be generally accepted to charge chock to chock either by time or some sort of meter, be it Hobbs or Tacho.
Our aircraft shows airborne time on the Dynon which is very accurate (unless you are doing land and taxi back to the hold) so we use that.