I use the front brake on just about any kind of surface, downhill, bends, outside camber, sand on the road. The trick is in your modulation. You can get very fine control of the brake, and ease off at the slightest loss of traction. Recovering from a front wheel slide is something that takes experience, but its quite do-able.
On one club ride, all the experienced riders negotiated sand on a downhill corner, including a couple on a tandem with a child on a tow-along. A newbie rider on a full sus MTB was the only one to wipeout.