I think you could make a good argument that you're more likely to have an accident due to leaving the towbar in situ than you are to damage the prop by pulling on it, especially if you pull close to the hub.
I've heard of a lot of accidents of this type, though none were fatal. I've never actually heard of anybody hurt in recent times whilst pulling an aircraft by the propeller, though I'm sure it must have happened and would be likely to injure the pilot. I've heard very few tales of non-wobbly propellers failing in flight or departing the aircraft or developing severe vibration which might be caused by somebody hand-bending the prop. I'd be interested to be hear if I'm underestimating the frequency of any of these types of accidents.
p.s. I realise that aircraft propellers and helicopter rotors are constructed very differently... Just trying to point out that dynamic loads are different from static loads.
According to:
aircraft-license.com/Demo/17.pdf
the centrifugal forces at the root of a 10kg blade at 2700 rpm may be in the region of 50 tons whereas the total thrust from the propeller is only likely to be in the region of a few hundred lbs. The resulting vector will therefore be within a fraction of a degree of the axis of the propeller blade.
It would probably be possible to design a propeller blade root that was very strong in tension and could resist torsion acceptably, but that could still be damaged by relatively modest bending moments that you might apply by hand. On the other hand, most aircraft propellers look pretty beefy. Without actually sitting down and doing an analysis (which is well beyond my ability) I don't know how you would actually answer the question of whether it is possible to damage them by pulling on the tips. Unless Jeff Capes is on hand to tie one in a knot.