I'm no professional pilot so i'll say this with novice ignorance.
I agree to everything that is said but what about in flight? The way I see it is if it was found half way through a flight that the pax count was out by one I wouldn't be concerned at all. Especially on a heavy where there maybe 8-14 cabin crew pacing up and down the length of the aircraft pulling and pushing trollies. What does that do to the CoG? Nothing that is noticable I imagine.
Just my 'outside of the box' thinking.