My mechanic lets me hang around and ask any questions I have.
This is not only during a tune but also during routine servicing.
This also give me chance to look at the car myself and have a poke around underneath the car making sure there isn't anything out of order.
Not only that, I learn a lot asking him about the car or when he points out stuff to me.
Honestly, I couldn't ever go to another mechanic who doesn't let me have full freedom around the car during servicing.