No less an authority than Sheldon Brown writes:

There is really no need for a rear brake on a fixed-gear bicycle. By applying back-pressure on the pedals, you can supply all the braking that the rear wheel really needs. In fact, it is fairly easy to lock up the rear wheel and make it skid, unless you are running a rather high gear.

If it's good enough for Sheldon, then it's good enough for me.