If you are descending a very steep hill or trail, you definitely want it in "4lo" and "1". That will get you the best traction and engine braking. Depending on your gears, you may not even have to touch the brake pedal.
When running everything but the steepest tightest trails, with the exception of down hill grades, I usually have it in "4lo" and "2".
In the sand dunes it's 4hi, and whatever gear keeps me going!
All this depends on your Jeep, tire size, and gear ratio.