Oh yeah, to touch on what the others have said, you should always try and scrub some speed before you get to the point of it being a MUST SLOW NOW situation. So do it before the you start going down or into a turn. Also, you have better control over the bike if you do not brake in a turn.

A good example of not braking on a decent, we have a wall of slick rock here that is approximately 25' high at a 75 to 80 degree angle and another that is close to 90 degrees and approximately 35' high. Any braking on those will make for some good photos and serious pain so you need to drop in slowly.