It's been discussed many times.
Yes, it is.
No, it is not.
The majority don't expect anything. As long as we pay them the standard rate and most important treat them right.
Can you afford leaving a little extra for a job well done? Then, do it.
How much? $10, $20, ..... $100. As I said, whatever. It's not like next time, they would spit in your food before serving it on your table!