Dear Robert et al.,
the one thing that would speak in favour of biting off the top end of the cartridge with the ball still in it, is speed. If one had to unravel the ball from the paper, and precisely aim where to spit the ball into, this would take up much more time than is known from contemporary reports. However, if one simply bites the lower end of the cartridge, pours the powder into the barrel, and then tries to stuff the ball IN its wadding paper into the barrel, it might not fit properly. We've tried this with live-fire drills. If the balls were cast so as to fit the barrels exactly, then the wadding paper, as an extra barriere pressed against the barrel, would make it hard to load a musket.
So it also depended on the cast of the ball that was used in a cartridge ;-)