Just stumbled upon this article. The author says:
"70% of bicycle crashes are from the front. If I had to choose between front and back lights, Iíd pick the front.
Think about this: a motorist approaching from behind has his headlights shining directly at you. Unless youíre a complete bike ninja with black clothes on a black bike and a black backpack, the motorist at least has a chance of seeing you. He has headlights to ensure he doesnít run over stationary hazards on the road, so hopefully heíll see moving objects as well. Even cars with broken taillights arenít invisible to following traffic."
Is it so? But usually the tail light is the most recommended and is required by law.