We have hills here (in Arkansas). So it's front and back brakes both on my fixie. I rode it the first couple of weeks around the flat parts of town, but eventually wandered into the hilly part. Going up not so bad, but coming back down...

As soon as I got 'er stopped, we went to the LBS and installed brakes on both ends ;-)

Front-brake-only can be a risk if you don't practice a good bit to avoid locking it up and causing an endo situation.