Hmmm, obviously a contentious issue this.
I was taught by my FIC instructor to turn the prop backwards. The reason given being that if you turn it the correct way then when the impulse magnetos operate, (which you can hear by the click they make) the engine could spring into life. I was under the impression that this is 'less' likely to happen if the prop is turned backwards.
(the reason why he was turning the prop at all was his request that props be left vertical to avoid the possibility of water pooling and freezing in the of the spinner thus causing imbalance/vibration)
Whopity, I'm no engineer which is why I am asking about this, but I find it hard to believe that the con rods are so delicate that they could snap when I turn the prop backwards, but I will certainly bear it in mind! Regarding the suction pump and potential damage, well if anyone can offer a technical/mechanical explanation of why this is so then it would be much appreciated - just so I can understand it all a bit better, (and incase any clever dicks ask why!)
So I suppose good advice for students would be 'if you absolutely have to turn the propeller by hand, do so in the normal direction of rotation, but expect the engine to fire at any time'.