It is surprising how sensitive the human ear/brain combination can be, in terms of picking out small rhythmic irregularities. Of course this is not news to musicians!
Over the past few days I have been practicing my sending, prior to getting on the air for the first time next month. I'm using PCW Fistcheck, which gives you a visual picture of your sending so you can tell how close it comes to the ideal.
Here is what my "Q" looks like, at the best of times:

If you look at the red "ruler" underneath the code, you can see that I am reasonably close to the ideal: 1 unit for the dit, 3 units for the dahs, 1 unit for the gaps (although the last dah is about a half-unit short). But to the ear, it sounds very imperfect. PCW Fistcheck is able to "copy" my Q with no problem (it recognizes it as the right letter) so I'm not doing too badly. Still, hearing my rather approximate code increases my admiration for the best code that I hear on the air.
By far the biggest tentation when sending is to lengthen the dahs, if they appear at the end of characters, words, or sentences. I'm using a straight key (the Ameco K-4, which is a J-38 clone) but I most often hear this "long dah" issue on the air when copying what I believe to be code sent with a bug (the dits are near-uniform but the dahs vary).
As a learner, I appreciate code that is as close to "standard" as possible. But I can see that possibly, a more "human" variant is fine too. In real life I am a typographer and graphic designer. One of the things some of us do when designing fonts is to introduce deliberate imperfections, so that the printed result looks less "computer-generated." This has its limitations (because with digital fonts, the imperfections introduced into a character are identical each time the character is printed) but there is some evidence that it reduces "reader fatigue."
I enjoy copying the ARRL code-practice transmissions, but one is also extremely aware that it is being generated by a machine! The slight imperfections (deliberate or accidental) that you can hear in code sent by even the best human ops is in a way reassuring, since you know that it is human-generated (and using a key, not a keyboard).
Here is my "B" (I am practicing my callsign):

Edited to add: BTW, great thread !
73 de Martin, KB1WSY