While I hate to disagree with Scott, I do
High Performance driving schools (at least none that I have been to) do not teach to down shift before braking, EVER.
Say you are going 140+ MPH at the end of a straight with a 180 degree curve ahead. You brake before you enter the turn to scrub off speed and shift the weight of the car from rear to front to help steering. When you enter the turn, you use the heel-toe technique (gas & brakes at same time) while you down shift into the appropriate gear. In some situations (never high speed) you could slow the car down by down shifting but you risk the possibility of over-revving the engine and it also throws the car off balance. Not to mention that brakes are MUCH less expensive than a clutch. There is a web site that details HP driving techniques ( I can't seem to find it now), I'll hook you up with the link.