Hammer down on an empty chamber is for Single Action Armys, Colt clones such as Ubertis and old model Rugers, etc. As far as I know, and someone correct me if this is wrong, any modern double action is safe to carry with a full cylinder. Also, modern single actions such as New Model Vaqueros/Blackhawks are safe to carry fully loaded as well due to the transfer bar.

S&W added the hammer block during WWII due to the incident with the sailor. They were safe to carry with six rounds long before that due to the rebound slide, though as mentioned that can fail if enough force is applied from a drop.

YouTube - Gunsmith - Safety features of the Smith & Wesson revolver