Have your LBS toss on a rear brake when your freewheel is installed. Extra stopping power is a good thing. You wouldn't ride a regular road bike with just one brake, right? Just my two cents.

People say rear brakes are bad for riding fixed because any back-pressure from the pedals in combination with hand-braking will cause the wheel to lock into a skid very quickly and easily, in my experience. Lots of people ride with two brakes and like it, though.