I'm strongly on 7HS' side regarding the usefulness of contrast tags. They're not "dumb" whatsoever, and if you think so you might as well petition to have adult and young removed. (But don't bother, they're not going anywhere.)
Should hair_up and hair_down be two of these? 7HS makes a good argument, actually, that other tags (bun, ponytail, twintails, etc) would theoretically be used instead for normal styles. hair_up and hair_down aren't the only way to describe those styles, so it's hardly a stupid idea to save them, or if not them then some other tag, for indicating style contrast. And that's the only time I would ever use them, myself.
The question is, is it "wrong" to use them on normal hairstyle or original character images? Well, I'd prefer for them to be contrast tags, but since we already have the well established alternate_hairstyle tag, it doesn't seem critical to have hair_up and hair_down restricted to indicating contrast. Especially because people will probably use them for normal styles anyway, diluting the usefulness of intending them for contrast.
So just stick to alternate_hairstyle. It's usually granular enough, can be combined with any hair style tag you want if you really need to get more specific, and you don't have to worry about policing a meaning of hair_up/down that some people won't agree with.