Overall I agree with Alchimista's answer, though I think it might be quite complex to grasp, seeing the comments that followed it.
If it helps, here's my down-to-earth version, which in fact covers chirality in general, not just cyclohexanes.
The 'executive summary' of my answer would be: not only it is correct to use an 'unnatural' or 'high energy' conformation of a molecule to look for its symmetry elements, but it's a fundamental part of the abstraction / rigidification that inevitably accompanies symmetry considerations.
Side note: the chloromethyl group in your example is not 'asymmetric' just because the Cl atom points to one side: this is just a side effect of the limitation of representing 3D objects in 2D. You could just as well write it with the Cl pointing toward you, and you would have 2 H atoms pointing symmetrically to the back of the group.
Symmetry is more fundamentally related to imagining the object in 3D (which I understand is not everyone's strongest suit, but nowadays you can find free 3D simple molecular modelling software by which you can explore these concepts).
Lengthier explanation below.
First, it should be clear that a molecule is usually an extremely dynamic entity, it never stands still, it keeps vibrating, and its bonds and the groups connected by them rotate in space in very complicated ways (just think of all the docking studies).
By such motions, the molecule will assume different conformations, some of which have higher energy and some lower energy.
However, between the extremes of these energy curves there are a multitude of conformations, and the molecule can spend some time in each of them [except of course those that are not physically allowed (typical case: atropisomers)].
I'm stressing this point just to clarify that there is nothing scandalous or outlandish in imagining the molecule in a conformation that is not the lowest energy one or a relative minimum: it is still one possible conformation, we are not breaking or forming any bonds to make it.
Even more fundamentally, the act of looking for 'planes of symmetry', 'axes of symmetry', etc., of a molecule, implies by necessity that we imagine a rigid abstraction of it: we are looking at it frozen in a state or conformation that is definitely unnatural for it, no matter how well we choose such state, even if it's a relative minimum in the conformational energy curve.
So it is totally acceptable to choose any conformation that is 'convenient' for studying the stereochemistry of the molecule.
Why? Very simply, because in any case if a molecule is chiral, you cannot possibly find any (physically allowed) conformation with a plane or centre (or more generally, rotary-reflection axis) of symmetry (i.e. 'symmetrical' in the common sense).
It follows that, on the contrary, finding even just one (physically allowed) symmetrical conformation guarantees that the molecule is achiral (so, meso if some stereoisomers of the same molecule in other configurations are chiral).
If you want to see this in a mathematical sense, you can think that the existence of one symmetrical conformation implies symmetry in the whole set of conformations the molecule can assume.
For each conformation A there is for sure a conformation A' (of the same molecule, of course) that is a mirror image of A.
In a more physical sense, once you have achieved symmetry in one conformation, for each molecular motion that breaks the symmetry, there is a corresponding, opposite motion that cannot be 'distinguished' (energetically) from the former.
On the other hand, if a molecule has no such symmetry in any of its conformations, this is necessarily never the case.
Here's a concrete example:
I'm sure you would have no doubt that this molecule is a meso stereoisomer.
Even if for some electronic reason the most stable conformer of this molecule were this:
which is not symmetrical, it would not make the molecule chiral, would it?
For any N conformations of the molecule you can imagine, there are for sure N corresponding mirror image conformations of it that are still the same molecule.
If you take this one instead:
it is sure that you will not find even a single conformation whose mirror image is still a conformer of the original molecule.