men who habitually wore armour, in many different cultures, tended to wear their hair short, whether they were crop-haired Romans, the 'pudding-bowl' of the European Middle Ages or the Japanese samurai top-knot.
So, I'd guess that the customary style among dwarfs would be either a short crop or a sort of 'mullet' with braids etc, along with a large, heavy beard styled to varying extents according to social status.
This is assuming that dwarf helmets are typically of a cap or open-face design. There is a definite historic link between closed helms and clean-shaven faces, so the generic picture of dwarfs wearing Scandinavian or Germanic type caps, with or without nose-pieces or short visors covering the eyes, is probably about right. There are numerous references to the faces and beards being visible, after all.
It might also be pointed out that although dwarfs are largely indistinguishable by gender, even among themselves, this isn't the case among humans for a variety of reasons. There seems to be a strong cultural emphasis on avoiding signs of gender differntiation. A human male adopting dwarf status might grow a beard, but a woman wouldn't; so, it might be customary for adopted humans to be clean-shaven.
but the information simply isn't there.