(From a vintage Ruger advertisement in my collection)
Old cowboy 'Single Action Revolvers' -- like the Colt Peacemaker used in the old West -- were carried on an empty chamber under the hammer because a powerful blow to the hammer (such as being thrown off a horse into a pile of rocks) could set of a round.
An old cowboy tradition was to carry a rolled up $10 bill -- known as 'burying money' -- in the empty chamber.
Nowadays modern six-guns -- like the Ruger Vaquero -- have a transfer bar safety, and some folks believe there is no reason to leave an empty chamber. I've been piled by my horse more than once -- so transfer bar or not -- hittin' hard ground is bad enough without havin' a hole in my foot too.
The way old timers loaded their six-shooter guaranteed they'd end up on the empty chamber -- the practice was to -- load one, skip one, load four, and let your hammer down on an empty chamber.
For modern cowboy shooting events folks figured out scoring was a bunch easier using 5-shots rather than 6-shots -- so transfer bar or not the cowboy tradition is carried on.