As BSMuppet says. you do need to look neat and tidy. In reality that means trying not to look like an accessory to the "Rock" industry. Even those individuals who are fortunate enough to run two careers, of which the "Rock" industry is one, still recognise the requirement for a conservative appearance when practicising professional aviation.
It is a conservative industry, and a conservative approach to grooming would be good advice. That aside, when you spend 12 hours with a headset clamped to your cranium, it does terrible things to your expensively coiffured and trained locks. Not a good look really.
In summary, keep the presentation smart tidy and professional, within the customer perception and expectation of the industry norm. Most people have a reasonable idea of what neat and tidy means, and the fact you are asking probably implies you already know a visit to the barber is looming.