Early cartridge revolvers (e. g. the Colt SAA 1873) had a firing pin that protruded enough to rest on the primer. Hard pressure or a hit on the hammer could cause a discharge. That is why in CAS you earn a DQ for the hammer down on a live round.
Hickok45 did a nice video on that topic: https://www.youtube.com/watch?v=ldHPNnsp-cs
More modern revolvers (e. g. S&W double action revolvers or the Ruger Vaquero) are built to keep the firing pin from the primer if the trigger is not pulled. They can be fully loaded without decreasing safety. It would have been safe to load all chambers of the Taurus.