Modern diesels are designed to achieve their peak torque power band at a low rpm. When approaching a significant grade I will often downshift the trans to 5th gear at the bottom of the grade. This will kick the rpm's up about 200 and spin up the turbo a bit sooner. My rig has a Cummins ISL9 450 HP motor. Here is an excerpt from my chassis manual on "Driving Tips".
When ascending a grade, maintain engine speed to within 400-500 RPM of governed engine speed. Governed speed will be 2200 RPM on the Cummins ISL engine model. Road speed may decrease, but the engine will be at its peak in the power curve.
On a minor grade I let the Allison take care of the shifting, but by anticipating a steeper grade and downshifting, by keeping the rpms up the rig doesn't loose as much speed before the Allison downshifts. This will also keep the temps lower as the engine isn't laboring as hard to maintain your speed up the grade.
It is especially pertinent to monitor your water temperature gauge when climbing steep grades. Keep in mind that it is not uncommon for the temperature to increase, especially in hot weather. If the gauge reaches the end zone or if the temperature warning light on the gauge panel should come on, reduce your road speed, shift to the next lower gear and keep your tachometer within 500 RPM of engine governed speed. In many cases this will stabilize the water temperature.
And since we have talked about how to properly go up a grade, this is how to go down the other side-
A good “rule of thumb” for descending grades is to never use a higher gear than was used to climb the same or similar grade. Try to keep the engine within 500 RPM of governed speed. This will give the best engine braking and reduce the need to use the service brakes. Select a gear that will keep you at a safe speed with minimal brake application. Never ride your brakes when descending a grade since excessive brake heat will build up and your brakes could fade, leaving you with little or no braking power.