How to select gearing for descent?
If You are DH racing, then we are talking about racetracks and trails that are known to the rider, assuming that the rider is practicing and getting used to the track, picking best lines for each section and generally speaking, creating/refining a race strategy.

In that context, a gear is to be selected according to the particular needs of each section, taking into account:


The entry speed for the specific rider on the section. 
The needed exit speed for the section. (Next section's required entry speed)
Length of the section.
Roughness and slope of the section's terrain.
The "tiredness" level of the rider upon arrival to the section.


Let's elaborate a little:

Entry and exit speed define whether you need to accelerate, sustain speed or decelerate. With that in mind, To accelerate you need a gear that gives a slightly slower than ideal cadence, because when pedalling the ideal cadence would be reached and then surpassed. This assumes only one gear is needed for the section. If more gears are needed, then the shift should be made as soon as the cadence is felt to be over the ideal. (The purpose is to be near ideal cadence as long as possible.) If speed is to be sustained, then select the gear for the rider's ideal cadence at entry speed.

Length of the section dictates if pedalling is even possible or if it means pedalling so long that XC pedalling technique is better suited. It also influences the number of gears needed.

Roughness also defined by whether it is possible to pedal or if it's better to focus on guiding the bike. Generally, the rougher the terrain the lower the gear. "Long" gears are better suited for smooth sections. The slope ash an obvious role: the steeper (downwards) the slope, the higher the gear, the steeper (upwards) the lower the gear.

Roughness and Length of the section together may also define if a "Pumping" or "Milking the terrain" technique is better suited for speed gain/sustain. 

Pumping is pushing down the bike over small descents in the terrain and pulling up in the small ascents. If the sections has many of these ups and downs, then you "pump" the bike up and down as you pass over the undulating terrain.

Milking the terrain is a more broad term that implies taking advantage of terrain irregularities to gain speed. For example, instead of rolling over a rock, use a previous rock to jump and land on the "descent" part of the next. The energy from the landing combined with the slope will push the bike forward.

(I know for sure that you can search videos for "pumptrack" and you'll find deeper explanations on these techniques).

Finally, the tiredness of the rider while arriving to the section play a major role on gear selection, even if overlooked. A DH race strategy has to deal with rider's energy dosage. Sometimes it is better to "rest" on a smooth section, to have arms and legs ready for an oncoming rougher section. Also, for some riders it is true that they pedal better on high gears when tired and better on lower gears, high cadence when fresh, for others, it may be the opposite, so, it is not the same if the "flat" section is at the beginning or at the end of the racetrack.

In my particular case, I tend to begin on a mid range gear, to accelerate faster on the race start, and sometimes keep a rather low gear because it is more likely that the pedalling section will have a slow entry speed. As I go trough these sections I progressively shift to higher gears and let it stay there, because I happen to pedal better on higher gears when I'm tired.

Of course, each trail or racetrack is different, so when I'm practicing, I note the gears I use, and try to decide whether each particular section is doable on a different gear. If I do my homework correctly, the race day I have cleared which gear should I use for each section. I do not memorize gear numbers, instead I recall what gear I decided was better for start and memorize the parts of the track where I have to upshift or downshift.

Almost forgot! Type of pedals may play a role too: I started to race DH on a XC bike with cleated pedals, I had been using them for XC for years, but when I got my DH bike, It had Platform Pedals. After some doubt and several races, I installed cleats on that DH bike and my times got a lot better in part because they allowed me to pedal faster (higher cadence) than the platform  ones. Also allowed me to pedal on rougher terrain and generally allowed for better bike control.