I found a reference in a book titled "Firearms of the American West, 1866-1894" by Garavaglia and Worman, p 346. "....John K Rollinson, who joined the M-Bar Ranch in Wyoming in the 1890's, wrote later that: 'We all carried guns. I remember that each of the six men had guns almost exactly alike. We all preferred the Colt single-action six shooter. Some liked the Bisley model, others the Frontier model. Some were of different caliber, but all were bulit on a .45-caliber frame. I noticed that these men carried their guns with one empty shell in the cylinder, and five loaded cartridges. This was for safety's sake. The gun was carried with the hammer on the empty shell'"
I too see no particular reason to have the empty shell in a chamber, but the fact remains, it was considered a safety issue in the 1890's to carry the Colt single action pistols with 5 live rounds, and is not an advertising gimmick dreamed up in the 1960's or later. If one chooses to carry their Colt single acion with six rounds, they certainly are free to do so, but should understand that it is not universally accepted as safe, nor is it a modern gimmick or concept to only carry five rounds.
"Far better it is to dare mighty things, to win glorious triumphs even though checkered by failure, than to rank with those poor spirits who neither enjoy nor suffer much because they live in the gray twilight that knows neither victory nor defeat." -Theodore Roosevelt-