Yes, downshift from D to 2 and to 1 as needed. HOWEVER, be aware that when you downshift, its like a manual transmission and you could over rev the engine.
On uphill, I usually downshift when the transmission is constantly doing upshift/downshift (Its probably more wear and tear on my nerves than wear and tear on the tranny) i.e. I'll downshift to 2nd, and hold the speed at 40-45 mph
On Downhill, if I find that I'm building up speed and need to hit the brakes a lot, I'll slow down, i.e. 40-45, then drop it down into second and let the engine do the braking.
Usually, I do what the 18 wheeler do as far as speed up or down hills. If they are climbing at 35 mph with the four ways flashing, I fall in line and do the same thing. Same downhill.
2001 Coachmen Mirada (Ford F53 6.8L V10) - Toad 2003 Saturn Vue.
It won't do MACH 2, but I can get a sandwich and take a pee.