FWIW, I would say try to figure out the maximum number of faucets you will ever have, then decide how you want them spaced. If your maximum number of faucets is an odd number, the center of the collar should be the center faucet. If it's an even number, the center of the collar will split the difference between the two center faucets. Install the faucets you have for now from the center out, and make a note of the spacing for future faucets. When its time for a new faucet, put an "X" of painters tape over the area where the hole will be so you can mark it easily and prevent some of the tear out.
I have 3 faucets on my kegerator, and have room for 1 or 2 more. I have a piece of tape on the inside that has the measurements listed so when I install another one, I'll know right away what the exact spacing is.
All that to say, I think you should drill the holes as you go.