I was thinking of the celts, the picts, the goths, the visi-goths and the scottish clansmen. Although, thinking about it, the picts rubbed ash in their hair to spike it so they don't count. I'm also pretty certain that the vikings and the huns wore their hair lose.
However, I've not really researched hair-styles through the ages - so my information could be wrong. I'd also suspect that hair is less of a problem once you've got sword/axes and shields anyway; so this could well be an irrelevance.