Always treat the prop as LIVE but in addition never get into or out of an aircraft with the engine running.
What's indicated here is a lack of understanding, and training regarding propellers.
You most certainly can safely manipulate the propeller by hand, and in some aircraft, it's absolutely necessary. From the responses thus far, one would easily assume that hand propping an airplane is no longer taught.
I grew up on airplanes without electrical systems, and handpropping was part of every engine start...no engine starter. I've handpropped airplanes over the years that do have starters when the occasion has arisn, and have always done it to teach students about proper propeller handling. Yes, it can be done safely, and yes, it can hurt you if you do it improperly.
Any radial engine pilot knows the necessity of pulling the propeller through by hand as part of the preflight.
As far as leaving the airplane with the propeller running, it's done in some arenas all the time. In ag work, for example, we frequently get out of the aircraft with the engine running. In the turbine airplanes, shutting down the engine and adding cycles is unwarrated and unwanted. In backcountry work, we'd sometimes get out, leave it on the left mag and chocked with a rock, and load our passengers and cargo. There are ways to do it safely and legitimately, but only with purpose and proper training.
Propellers can certainly be dangerous, but no pilot training is complete without being properly trained in the safe handling of the propeller. It is wholly inadequate and irresponsible for an instructor to merely teach a student to fear the prop.