Look, there's obviously a lot of opinions here, here's my 2 cents.
Man has advanced over thousands of years to where he is now. Do you think that evolution/God meant for man to have access to a great shower/large waterfall 24 hours a day? I think not. It is NOT essential to wash every day, either your body or your hair.
However, in this modern day that we live in, it is indeed nice to shower daily (I know people who shower 3 times a day and that is obsessive). You won't die if you don't shower for a week, but you won't attract any hotties in this present day. Incidentally, I love when I can smell girls just when I am talking to them, its a shame that smell/pheromones are washed away these days.
As for hair, those oils aren't bad for you. How could that be? We are born with them and produce them till the day we die! They are the same oils in an animal's coat that keep the coat healthy! But have you ever seen an animal with great hair and no dandruff? And I'm not counting horses here that are cared after.
So, there is no danger from not washing much. Personally, I was my hair and my body daily. It keeps the dandruff away.
And actually with dandruff, I find that if I don't wash my hair for a few days, then dandruff will build up. I wash my hair. Guess what? there's even more dandruff! I touch my hair and its like Christmas or something! I use head and shoulders but the key here is, wash once, then REPEAT! That repeat really clears the dandruff. I don't repeat every time, but every once in a while it does some good for me.
Incidentally does anyone know how to get rid of eyebrow dandruff/flakes? Don't really want to put any chemicals near my eyes....