Most Doctors, from what I recall, maintained the same hair-do throughout their entire 'lifetime' on the show.
Peter Davison had a haircut in his second story - and grew it back again for the next one. (Something to do with transmission-order!)
Colin Baker just let his grow ( like Jon Pertwee did).
David Tennant went from short to very errrm spiky!

Someone managed to write two entire books of continuity errors on Star Trek:The Next Generation... If the least we have to worry about is the odd person switch and a hair error we're doing well.

Ummm didn't notice was too busy watching the epesode.