I use my front brakes most of the time. With a little practice you learn how to balance between stopping and flipping. As was mentioned before the front brakes provide the most stopping power, by using your front brake you avoid skidding your back tire which tears up the trail and hurts you ability to stay in control. Practice a bit with your front brakes and you will start using them most of the time, the trick is to make sure you keep your weight back.