After seeing a video on a video sharing site, regarding sheep shearing, I decided to look more into it. Shearing has been around for ages, and as I understand it, modern shearing methods & technology are generally safe for the sheep, though no technology or method is perfect of course.
Human error (or technical malfunction) can cause accidents & injury, but the benefits (to humans, in regards to resources, and sheep themselves in regards to health & wellness) are well known to outweigh the risks. I would never advise anyone without proper training shear a sheep, at least not without professional supervision.
This of course is common sense.