I think it depends, Im short and with a strapless dress and my elbow length hair, it makes me look shorter, if your blessed with height you hav total freedom because either way you wil look great. If your a tall person who doesnt like looking tall (my friends hav experieced this), maybe your hair down. But i think hair up looks great because ur neck looks long and ur shoulders are showing off.
Good Luck

XOXO.