I'd like this option too, and was surprised it isn't in the game already. The first helmet I picked up for my MC -- some leather thing with tassels on the back -- was hideous.
Ideally we would have three options: always shown, or shown except in dialogs, or always off.
I like always off, even if there is some cognitive dissonance in gaining AC and perks from helms that don't show up in-game. It makes it easier to always identify my party members. The look is consistent whether they're talking in cut scenes or out in the game world.
I have recommended this before but they should not only allow you to hide your helmets but also hide or change the look of any of your armors.
A lot of games do this in both single player and MMOs. You can choose how your armor appears and are not forced to change your look just because you got an upgrade.
Assassins Creed for example.
If anything they should go further. We should be allowed to recolor the armors in the game as well. Go to a dye shop and recolor those brown leathers black! Because why not!
Glad I'm not the only one. Just wanted to highlight the last two since you both put up great examples that would be nice to have as well. I think both great suggestions to be considered and added. I am sure we are only getting the less flashy armors and cosmetics right now since it is still in development, but I hope they take your suggestions into consideration. I think it would be awesome to be able to have those options available.