"Hold your Position" almost always occurs once clear of the runway. I sometimes don't get it if I am taxiing very slowly at that point. It was always the same in PFE. So perhaps it's way of identifying your precise position.
Absolutely correct. On larger airports in particular it can take a little while to calculate the best taxi route, in which time, if you're taxiing too fast you could have already past a few of the taxi waypoints that PF3 has decided to use. That in turn would mean you have to hit the 'skip taxi waypoints' hotkey a few times. So it's always best to turn off the runways and then slow down to a crawl.
