I guess I got lucky!
My dad is a mechanic/electrician and taught me pretty much how to do everything at a young age.
I do occasional rewiring on classic cars and tips are freaking AWESOME.

I suppose it depends on the relationship you have with your mechanic, if you're really into cars then you usually have a pretty good bond with them.
Best way to decide: if they did great work, and you plan on going back, tip them! It's pretty much guaranteeing that you'll get future great work from them.
Even if they refuse, it's still a nice offer