Why do you guys want to cheat like that? You're only lying to yourself when you artificially inflate your power figures like that.
I think the main problem would be heat soak when you sit there with your car idling before the run.. Hence the dyno figure might be artifically DEflated, because this is also not the normal condition that a vehicle operates under (in most occasions). You could do the run without warming the car up first, but who wants to subject their engine to that? I mean.. some people don't even like the thought of starting their car when there's a bit of ice on it from overnight..
Lets not forget that there ARE trophies involved also.. That might be incentive to use every little advantage for a higher figure (whether it's accurate for day to day driving or not)