The factors to consider in this equation include:

--  Terrain;

--  Brake capabilities:  To stop & To cool;

--  Transmission type: Manual or Automatic;

--  Weather conditions;

--  Engine management.

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Overall:

Think before doing any driving maneuver that is out of the ordinary for you. Become familiar with shifting and don't drive NASCAR around your hilly areas.

The primary concern is to drive so that you can maintain control of the vehicle. Your downhill speed must be controlled with a margin of safety to allow you to come to a full stop if need be. You must be able to fully stop if something unexpected occurs.

Wear on the drive train is far less important than safety.

Terrain:

In terrains in locales such as San Francisco and Montreal, this question is important. If you live in less-hilly country, it is less critical, but interesting to use appropriately.

In all cases it is interesting to consider and it may result in improved safety along with reduced drive train wear.

Brake Capability:

Brake capacity to adequately cool and adequately brake for an extended period will vary greatly by vehicle type and model. Also, if your vehicle is loaded with extra weight, its brakes will be required to work harder than if it were carrying just one or two passengers.

Learn your vehicle's braking capabilities by asking a mechanic who is familiar with your brand and model. Sports cars have relatively more potent brakes than small econ-o-cars. Mid-sized sedans should be judged specifically.

SUVs are generally the heaviest vehicles and therefore need the most resilient braking systems. Check out your specific SUV to identify its capabilities. A loaded SUV may have brakes that may be stressed to their limits on a long, steep incline.

Transmission Type:

If you drive a manual transmission vehicle, it is easy and a natural part of driving to shift up and down. In this case down-shifting is recommended if you are comfortable doing it at the time. The engine serves as a brake and reduces brake heat build up and wear. Care should be taken to prevent the engine from over-revving.

If you drive an automatic transmission vehicle, shifting is less a normal part of driving. If you know you are approaching a steep downhill stretch, you can down-shift while traveling at a slow speed with low torque before starting downhill. This will minimize strain upon the transmission.

Weather Condition:

If it is icy or wet you should take extra precautions to not shift while navigating the down slope. Jerky shifting can cause the tires to lose traction which may result in a dangerous loss of control.

Engine Management:

This also depends upon your specific vehicle's computer control and its engine's mechanical capabilities. Recall that safety is the primary issue. If down-shifting "confuses" the computerized engine management and causes a stalling condition, avoid it.