The guys that really bit me were glow converted Zenoah gas engines. Those were the days before I learned the backflip method on glow engines.
Gas engines are quite well behaved, as long as you don't play around with the prop. Whenever you touch that prop, either grasp it real tight, or flip it decently. Some engines even need a lazy flip to start real good, like the early MVVS (evolution in USA) with Vlach ignition. RCexl ignitions need a decent flip. I never had one hit back on me, so a decent flip is good enough to retard the ignition.
Eh.. no, I do not wear leather gloves for starting. Too clumsy. If a prop decides to kick back, it will swing back my fingers only without doing any harm. Never happened though.
When preparing the plane/engine for starting, do so with ignition off. That is what that little switch is for.
And like AV8tor said, sharp edges on props have no use for them.