so you don't know how many POB you have when you taxi?
Correct, we do not have final POB when we start taxiing. We do have the provisional figure.
Or more pertinently you're happy to close up and taxi without having that info?
Personally as long as I have provisional figures I'm happy enough. I suspect many other captains and just as importantly many other operators and their regulators are "happy" to do much the same thing.......
What happens if you need to evacuate the aircraft and do a head count?
Is the POB that important in the first few minutes? What happens if the flightcrew and loadsheet don't survive?
Being optimistic the crew will give the provisional figure to the incident commander.....whether he/she is interested in an "on the nose" POB whilst folks are still coming down the slides is only something they can answer. In the real world in the event of an incident involving a full widebody I suspect by the time passengers and crew had been rounded up, led away to safety and a head count could be performed in a controlled and meaningful manner the final POB would be available to the authorities.