Well, not to pick at nits, but I don't think it's totally personal preference any more than the best method for bowling is. There are good ways and bad ways, and the good ways provide better (and more consistent) results for most people.
The closer you can get to the middle of the pad of your trigger finger the better off you are. For DAO triggers, it is easier to pull the trigger when near the joint, but unless you practice, practice, practice I'll bet you pull to the right if you position at your finger joint.
Fit is a consideration, too. If your finger is on the trigger and the gun is a good size for your hand, then there should be a bit of daylight between your index finger and the grip of the gun (one instructor I had called this the pencil test because the space should be about the right size to slip in a standard pencil).
Cheers,
ChickenHawk