Regardless of the semantics of "should" or "must" the end result is that unseen damage has been known to occur when control surface are permitted to flail unrestrained in strong winds. Examples were given in earlier posts. The fact also remains that complacency causes accidents and what on earth is this perceived insurmountable problem with taking the commonsense precaution of chocking the rudder of a light aircraft to avoid the risk of damage. The reasons given to restrain rudders and control surfaces are well known. The trend in recent posts is to find reasons why control surfaces should NOT be restrained.
If a student is seen to rapidly and viciously apply full control wheel and rudder movement hard against the stops during the pre-take off check of "Controls Full and Free movement" you can bet the instructor would say hold on a minute mate - why the bashing of the controls to full deflection like that - you could cause damage. And of course the instructor would be quite right. So what's the difference between a student abusing the controls - and a strong wind doing the same thing with the same potential risk of damage? That is why all surfaces should be restrained when the aircraft is parked especially if strong winds are possible.