@ testpanel, OK I could have phrased that better - I meant that I was not talking specifically about 737s, about which I know very little. How do you check the fan on 757/767 then ?
We were once told about an engineer who was working on a fan engine, removing the P1 probe. A part of the probe assembly fell back down the mounting hole and he put his finger in to try to retreive it, upon which the fan, (which was rotating due to the wind), cut his finger clean off !!
Perhaps this is what you were being warned about, but there is no such danger from gently grasping and turning a fan blade from the front - (just don't get your skin jammed between the tip of a blade and the abradable seal).
If, due to the wind the fan is turning any faster than very slow, don't touch it at all - there can be a lot of energy in a rotating spool and you could damage your hand.
As for damaging the acoustic lining, I don't know about Boeing, but on Airbus you would have to do something extreme to even make a mark. What the hell was your instructor doing?
In my experience, walkarounds are notoriously badly taught - if they are even taught at all. (In all my Airbus flying I have never been shown what to do by anybody), but your FCOM should have detailed instructions about what to do and look for.