I decide what to constrain based on what I want the cloth to do, and what type of cloth it is. For dresses and skirts, I only constrain the top row of vertices. For something like a body suit, I tend to constrain the whole thing. Different people will tell you different ways. Experiment with it and see what you can come up with.

If I had a nickle for ever time a woman told me to get lost, I could buy Manhattan.