I think one way or the other personally. I usually look at the waterfall and decide what I want the outcome to be. If the light and scene it self is serene then I'll usually choose a slow speed and go for the veiled look. If I want to really show the delicate nature of the water (usually in a closer shot as opposed to a wider composition) I'll ramp up the speed and freeze the water. If I want to represent the scene as close to how I saw it then something in between usually works.