Yes, you'll want to constrain it so it stays up. Check out the dynamic dresses at PoserFashion.net. The "Great Ball Gown" is contrained around the neck, to keep it from falling down. If you look at it in the Cloth Room, you can see it. Just click on the "edit constrained group" button, and the you'll see all the constrained bits in red. To keep the drape natural, you'll want to constrain it at the places it where you wouldn't expect much movement, and only a few vertices here and there, not huge solid blocks.