I always downshift... really helpful to have it as a habit when you go canyonrunning (fuck 'touge') for fun on weekends... need as much power available as possible when you're doing a hillclimb, especially at altitude. Otherwise you just bog on the exit of a turn; and when you're trying to keep up with cars more than a half ton lighter than you, popping into the wrong gear and feeling the car struggling to get into its power range just *sucks*.
Same thing on the downhill... our cars have excellent engine braking, and it helps to delay the onset of fade for quite a good little while, even with the major weight disadvantage, and crappy one-pot '90 front brakes.