It's about how much the stern is depressed into the water when underway. If the hull makes a clean exist from the water on it's bottom using the rocker then there is no rooster tail. If the hull makes an exit from the water with the square stern, then there is a rooster tail. However, I wouldn't imagine that the presence of a rooster tail isn't necessarily a bad thing. As long as the water is leaving the hull cleanly (i.e., the boat has enough hull speed that the water isn't flowing backwards into the stern) the rooster tail is no real issue.

You also have to remember that things are a compromise between varying wind conditions. In light air, we move forward to keep the square vertical part of the stern out of the water (among other reasons). In heavier air there is much more pressure on the buoyancy of the hulls due to the leverage, we move our weight back on the boat to keep the bows at proper trim which pushes the sterns down...which isn't much of a problem because the sterns are exiting cleanly at that point.