Never hang a medium or heavy weight sweater as it will stretch. A thin cashmere sweater particularily a cardigan can be hung but use only a wide shouldered hanger. As a thin sweater is much lighter in weight, it may be ok but this is still not recommended. Better not to get tinto bad habits in the first place.
I've made the mistake of hanging sweaters before. Fortunately, they were cotton and regained their shape upon washing. Now, I would never hang, except by the method just posted in here. That said, I'm still gonna be folding- the shelves in my closet are optimized for such, and that stack of colors is just pretty. And it also gives you a great idea of the character of the sweater you grab, rather than having to shuffle hangars around.