Jon, the amount of fill flash that is needed is most certainly determined by the camera setting! Aperture determines a flash unit's maximum useable distance. If you're photographing a group of people outdoors at f/16, you have to be closer to them to get the same amount of fill light that you could get from a greater distance at f/11, since it's likely that a camera-mounted flash would be putting out full power to cover that small f/stop and distance.
One of the things I like most about my D1X is the low 125 ISO and high sync, which lets me shoot fill flash at 1/500 at f/8. I can shoot fairly large groups of people even 15 feet away and still get fill from the flash.