I wash my hair everyday, sometimes twice if I get dirty or sweaty, and I've never gotten lice.
I just don't like to leave the house without having showered and washed my hair. However, I've met a lot of through hikers on the Appalachian trail over the years and they say a lot of what is being said here. These are folks who backpack 15+ miles everyday for 7 months or more in order to complete the trail. They say your body (and your hair) goes through a funky stage when you stop washing with soap every day, but then it sort of finds an equillibrium and you stop stinking so badly and just get a kind of mild musky smell.
I just can't bring myself to do that.