I always have problems with making helmets invisible as a helmet is the most important piece of armor by far and only very stupid, inexperienced persons or Hollywood stars would go without in a real combat. So I don't use the toggle.
However, it's a game and many helmets really look strange or distorted (a lot are too small) in DOS2 (only like the most of the elves helmets) so it's a good feature you can get rid of them visually.
