Do not pull near the end of the blade, the prop is at it's thinnest and you have the longest leverage.
The most thrust is generated by the propellor tips, as they rotate the fastest, so to suggest that the half horsepower or so that you are able to generate by hand would be sufficient to bend the tips... I don't believe it.
Nevertheless, there is no mechanical advantage to pushing or pulling on the tips, over pushing or pulling on the root, so use the root. Better safe than sorry.
Another contentious issue is whether you are allowed to rotate the propellor so that it's horizontal (and thus clear of the tow bar) or not. And if so, which way to rotate it.
Some engines (Thielerts come to mind) do not like to be rotated by hand whatsoever, because the gearbox now works in the opposite direction. Other engines, including the traditional direct-drive avgas Lycomings/Continentals fitted to most of the GA fleet, can be turned by hand. But I have not seen a clear, official statement from the engine manufacturer regarding the direction.
Some argue that you should always rotate the prop backwards. Reason is that this reduces the chance of the magnetos firing with an air/fuel mixture in the engine. Others argue that you should never rotate the prop backwards, as the ancillary stuff bolted to the engine (such as the vacuum pump) can be damaged by rotating it in the wrong direction.
To be absolutely on the safe side, as a rampie, ask the owner about the correct procedure/rotation direction and stick to it.
Furthermore, three more tips:
Never put a body part inside the propellor arc that doesn't have to be there. And if you rotate a propellor, do it so that if the propellor accidentally moves on its own, your hand will leave the prop arc by its own momentum.
Make it a habit to *always* check the mixture and magneto switches before going anywhere near the prop.
Never leave a towbar attached to the nosewheel assembly when you leave the vicinity of the aircraft. Disconnect it and put it across the nosewheel path as an improv chock. Starting an engine with the towbar still attached may cause major damage, not just to the towbar, but also to the prop and innocent bystanders.