I like this sketch by illustrator and character designer Peter de Seve.
While lesser artists strive to get symmetrical features correct, you can tell de Seve views "symmetry" as the waste of an opportunity to squeeze more character into a drawing.
For example, there is nothing uniform about these two wings:
Or the two sides of this hat:
Or these two feet:
|Note how one foot is large and defined while the other is small and feeble and dribbles away, just ike the man's life.|
And certainly there's no symmetry in those marvelous teeth:
Essentially de Seve has drawn each side anew; there are no mirror images here. That means twice the drawing work, but also twice the opportunity.
Or, note the tail on the creature. Where de Seve doesn't require a tail, he doesn't even bother to complete the outline, but where he really wants one (that curl at the end) he comes back to emphasize it with some of the thickest, darkest marks in the entire drawing.
As another example of good drawing priorities, look at how the fingers below are just a clenched jumble of lines (how many fingers can you count?) yet the knife which commands our attention contains descriptive details such as a that blood groove or the shading along the underside.
De Seve doesn't waste these sketches; there's a lot of thinking going on here about what the picture really requires and what it can do without. And once he forms conclusions, de Seve is one artist with the technical ability to implement them.