Its not just the prop that it may harm.
The bearings arn't designed for an off center load so if you just pull on one side you are squishing hardened bearings into the soft race which will start giving play on one side if you do it often enough.
The engine is designed though to have the force through the shaft so the closer you are to symetric the better so a hand either side close to the hub won't do anything but a pull at the tip one side won't be healthy.
The complexitys of the stress level down near the root in flight is completely different to a static point load. Props just arn't designed to be point loaded they are designed to have a pressure applied over the length and the pitch is altered to vary the pressure. And also to take the centripetal forces of it spinning.
Also a symetric straight pull is OK but when folk try and steer by pulling one side or the other it won't be doing any good.