Ok, first off as far as drag marks and scratches in the cylinder the working cowboy usually didn't worry about such things as we do today. I have read about them using revolver butts as hammers for fence staples and twisting barbed wire around the barrels. They were tools, nothing more. As long as they were clean and went bang, that's all that mattered. Not that all the old timers felt that way but some did. Second, people back then that expected trouble probably thought the extra round was worth the risk.
Now to the OP. The first reference I have read was from a man named John Poe. He was one of Pat Garret's deputies when Garret shot Billy the kid. Poe stated that after hearing Pat's 2 shots at the kid, Poe swore he heard a third shot. After examining the kid's revolver they found only five live cartridges, but the spent caseing did not seem to have been fired recently AND that keeping only five cartridges in revolvers was common practice. Poe's first telling of this was in 1882, 1 year after the Kid's death. My .02