Well... before he was Sisko, Avery Brooks played an iron-fisted mob enforcer named Hawk, and his preferred look was shaved head with a goatee... when he was hired to play Sisko, the producers presumably wanted to distance him from that admittedly iconic character. So, it was written into his contract that he shave his chin and let his hair grow. By Season 4, when it was time to renegotiate the cast's contracts, Sisko was more established in his own right, and Brooks was able to renegotiate that part of his contract. This allowed him to look the way he wanted to look.
I'm normally not wild at all about shaved heads, but I have to say, Avery Brooks made it work.
