I always ran a front brake on my fixed gear. No rear wheel brake will stop as well as a front wheel brake. That's just physics, and the laws of physics are always strictly enforced.
I'm also running a rear brake now. Once I moved to a hillier neighborhood I found that I could descend faster when I had the rear brake to keep control of my cadence. Without the brake, if I spin up to the point where I'm bouncing in the saddle a little I have a very hard time getting it back under control.
The rear brake on my fixie wears out much faster than the rear brake on my freewheel bike.

em