I grew up having that drilled into my head, too. I'm pretty sure that it was because it would have been such a nightmare for the Home Ec teachers to have to re-adjust all of those tension knobs, after every class, if the girls played with the knobs during class. So, they scared us about it and there is a whole generation of sewers still afraid of "touching the tension knob". lol
Now that I collect and repair vintage sewing machines I see that it would be very difficult to destroy a tension knob or even a bobbin case. I can even take them apart, clean and polish the parts AND THEN put the whole thing back together again!!!! The internet is an amazing place to learn all kinds of things.