This precedent was established upon the release of Metallica's Load album 20 years ago. You can dislike the Black Album, as well you should, but it is still metal. Load is not metal. Never was. Never will be. Notice the short hair transition betwixt those two albums. Point: proven.
The only exception I can possibly provide are ISIS. However, one can argue that they, in fact, are not actually metal. I wrote that sentence before reading the thread. I see that I am late to the party. Neurosis are old man metal + the heaviest matter in the universe, therefore they do not apply to the rules of us mere mortals.
Also I happen to be listening to Tempel right now so Agalloch, you are correct. But... you guys would be better with longer hair. I'm just sayin' try it out, and odds are your next album will be even better.
Friends don't let friends get haircuts. I'll rock the skullet in my olden days before snipping my frizzy locks.