I do not do a lot of sacking out as what you would think of as the norm. I start them once they hit the ground learning how to react when something scares them.
How exactly do you teach them how to react, I think every horse reacts in a different way anyway. I had one gelding that would literally jump when he got spooked, the Mare I'm riding right now reacts totally different- a couple of steps. I do try to sack them out, but as mentioned above, you can't prepare for everything.