You want to stay on top of the ball and not cut to either side. That's how I've been taught. To get a 2-seamer that breaks toward the same-side batter (and a little downward) you want to apply more pressure with the index finger.
During one of the recent playoff broadcasts they had John Smoltz on and he discussed how to throw Greg Maddux grips his 2FB. Your arm naturally prontates, and by adding extra pressure on your index finger you release the ball with a bit of pronation, which gives the pitch its characteristic movement.