Wild Bill Hicock with cavalry style holster, from the SMSU Department of History, Springfield Missouri

and another image, from the collection of the Smithsonian, also with a cavalry style holster arrangement:




and this one, where both holsters are more clearly visible,  second down, far right column:


One of the nice aspects of these various photos is that they clearly show Wild Bill Hickock's consistent pattern of hand gun carry over a period of time. While he sometimes is shown with a single revolver on one side or the other, this may or may not reflect a mirror effect with these images, as has been noted in other images from this time period, notably Billy the Kid.

But what it does show is that more often than not, Wild Bill is shown with a long barrel weapon, and when he IS shown with handguns, it is consistently done cavalry carry, revolver butt forward, worn higher on the hip not in a holster strapped to his thigh, and not in the sloppy, dangerous, uncomfortable and inconvenient manner of being shoved through the front of his belt. For anyone who routinely works in rough country or on horseback to favor sticking a gun through the front of their belt would be ludicrous then.....and now.
There are multiple photos of Wild Bill Hickock showing a cavalry style holster, sometimes on the right, sometimes on the left, sometimes with two pistols simultaneously.  There are other, additional photos where it appears that he is carrying in a holster, but the gun and holster are under his coat somewhat, and the clarity does not make his use of this specific kind of holster as clear as these images do.

Hickock had a very distinctive face, and he appears to have been photographed often.

Anyone who has spent an amount of time around horses knows that riders mount from the horse's left, known as the near side, swinging one's right leg over to the cantle of the saddle and the back of the horse to the right side.  The right side of the horse is the 'off side', and it is a rare horse that is trained for a rider to mount or dismount from that side.  In the course of riding, particularly over any kind of natural terrain as contrasted with a flat, paved road, will find themselves bending frequently from the waist, whether it is in the course of jumping small obstacles on horseback, riding uphill where one leans forwards, or having to lean forward to either open a gate, make changes to a fence or cart from horseback, or to use one's roap, or to reach for the long barrel rifle that is customary in the old west, as well as any number of other activities such as checking the saddle girth, reaching for a canteen, etc.  In addition there can be numerous occasions to mount and dismount.   Of the two sides of the body, the right side would be even more uncomfortable and more unsafe for a 'mexican' or belt carry, than the left side; but neither side, nor the front center would be practical, much less desirable.

Anyone who rides, anyone who has clambered over the rough terrain of the American west that remains in a wild state similar to the old west, would recognize immediately how inconvenient and downright uncomfortable it would be to have a firearm stuck in their waistband or belt.  Any gun carried in that fashion would be in the way, and would be a danger not only to the person carrying the weapon, but also a danger to the rider's horse.   Cavalry carry holsters developed precisely BECAUSE riding caused weapons to bounce around dangerously if not secured.  Now I have spent some time in the saddle, in places like Montana, on working ranches.  In my experience, working real-world ranch hands, as distinct from gun loon fantasies, are very careful to keep their equipment (including firearms if any), secure.

From wikipedia on cavalry draw / cavalry style carry:

"Cavalry draw is a method used to extract a pistol from a holster, which is designed to carry the pistol butt-forward. The name and technique come from the gunleather used by the cavalry of both the United States Army and the Confederate States Army, during the Civil War. The pistol was in a covered holster carried high on the cavalryman's right side, but was placed butt-forward for crossdrawing by the left hand. The pistol was considered by the Army to be a secondary weapon, with the right hand used for the saber. Placement on the right permitted an alternate method to be used, allowing the right hand to draw the pistol if the sword were lost in battle.
In practice, however, the "alternate method" became the standard, with the sword being left in its sheath until the pistol and its spare loaded cylinders had been expended.
Later, it was found that the reversed holster can be more comfortable, especially when worn while sitting down, than the normal type holster. In addition, cavalry draw can be performed while sitting, as well as retaining the original off-hand crossdraw capability."

Wild Bill Hickock had a clear preference for the butt-forward cavalry draw, and for wearing a variety of holsters to facilitate that. as shown in collected photos and daguerreotypes of him.

This is in contrast to a couple of photos of him with belt carrying unsheathed knives and revolvers, or with a partially drawn knife, that appear to be staged for publicity photos as distinct from representing his actual habits of gun carry.

About the only way one could carry a firearm that would be more dangerous, or more stupid than belt carrying a revolver, to someone with experience riding in the western states, would be to hang their revolver from a string around one's neck, strung through the trigger guard, like some demented and dangerous lavaliere with the barrel pointing upwards.  I'm just waiting--- waiting -- to come across a photo of one of our gun lunatics doing so, with their favorite manly fetish toy.

One of the lessons of history that I took from traveling through the western states as a child, including touring through museums, was listening to the tour leaders and history guides talking with a mixture of humor and disgust  about how much myth and misinformation existed around wild west figures, like Hickock. 

Apparently that hasn't improved with time.