Since 1945, Smith @ Wesson revolvers have had not one, but two, independent safeties. The first "rebounds" the hammer, causing the hammer (regardless of where the firing pin is mounted) to rotate backwards behind the breech face. It then blocks the hammer from moving forward. The second moves a steel block up to just below the firing pin and places an additional restraint to prevent the hammer from moving forward.
Unless someone has tampered with the system the cylinder can safely be loaded with all the chambers filled.
The only exception I'm aware of is those models with a fully enclosed hammer, in which case the second hammer block is eliminated because with the hammer fully covered by the frame it's unnecessary.