I find it offputting if a mechanic doesn't want me watching him fix my car.
I want to be sure everything is done correctly!
If you want it done right do it yourself, or double check others are doing it properly!
Yes. They've never said no. I ask about their tools, their technique, etc., and they seem happy that someone is interested.
You hang out in their shop and watch them? They allow this?
I always ask. I myself hate working when someone is standing over me watching, so it only seems right to ask for permission before doing that to someone else. Basic politeness is free.