It depends on what you're using the mirepoix for, including what size you want, how long you're going to cook the dish, and how much juice can come out in the prep. I'm assuming from the question that reasonable, but not fanatical uniformity is the goal. If you're making a soup or stew without caramelizing the mirepoix or if you're going to simmer the dish for a longer time, you have more flexibility in how the flavors can come to the party. At the other end of the spectrum, if I'm doing something like a stir fry, I'm much more careful in how I chop. Nathan (volume 2) has convinced me that smaller pieces release more flavor, so I'm cutting things much smaller these days. Most often I'm looking to practice knife skills, but otherwise I pulse each veggie separately, as a couple of folks have mentioned. The Cuisinart does more violence to onions (and shallots) in realeasing juice, which you typically don't want to happen early, so I generally do those by hand. In contrast, I've had good success with pulsing the Cuisinart to get a fine mince of garlic.