When you get your head shaved, does the barber start down the middle?

I'm tired of having my hair long in summer. It gets so hot having it, so this summer I'm going get a #0 buzz. At a barbershop do they begin at the top and go right down the middle when they start? I want to make sure I don't change my mind about getting it all buzzed off. Is the #0 the shortest I can go? Does it take long to get it all buzzed off?