Probably not a good idea to watch cop shows, that aren't taped from real officers, and expect it to be similar.
We sometimes watch them in the break room to laugh at how the tactics are so very far from real.
However, how a light is held with no gun drawn is determined on the situation. As explained earlier.
Here a few sites that show how to use a gun and a light. Even shows how to use side switch lights.
http://www.surefire.com/articles-handheld_techniques
http://www.nrapublications.org/si/HB_handgun.html
http://www.surefire.com/grahamMethod
Note* The Graham Method, I do believe, was created by a US Air Marshal.
The Graham Method and the Rogers/SureFire have been the best all around in my opinion. Once you get used to getting into the right positions quickly.
Rogers/cigar hold is fine as a gun/light technique, but IMO it is a poor general purpose technique. Also, it doesn't work well with recessed tailcap buttons.
Graham IMO is wobbly to aim. You are trying to mate a small, slightly curved surface (tailcap button) with another small, irregularly curved surface (knuckle).
The NRA Publications discussion of the various flashlight holds is not the best way to actually learn how to execute the various techniques. It is a nice overview, but leaves out a lot of key details.
For example, the Ayoob method discussion omits some important details and is incorrect in another.
The Ayoob Method is good for short-range gun-light usage. Beyond about 7 yards, the light typically is angled too much so that the beam goes over the opponent's head. Also, more specifically, it isn't just thumb meets thumb. It is base of thumb meets base of thumb.
Isometric tension is not what stabilizes the gun in the Ayoob Method. Firearm recoil always moves the gun toward the path of least resistance. For a one-hand gun hold, that means the gun tends to move toward the open "C" of the hand. That happens to be where the flashlight hand happens to be, and that is what stabilizes the hold.
The discussion on Chapman neglects to mention that the method is slow to acquire. The grip isn't just a generic "sword" grip. Specifically, I'd call it closer to a sabre grip, with thumb forward. There are some technique details that help align the light with the gun. You want to use the "OK" finger sign and encircle the light with the thumb and index finger. align the flashlight tube parallel with the long axis of the thumb. Where the thumb points is where the light points. The other 3 fingers grip the gun in a modified two-hand hold. Most people can operate the light using their thumb.
For Harries, the safest way to get the flashlight under the gun and to the other side where you can mate back of hand with back of hand is to index the flashlight hand against your torso and then along the outside of your shooting arm. Body contact will pretty much eliminate the possibility of crossing your flashlight hand/arm with the muzzle.