[Gonso]

usually:

1) taxi drivers, also valet drivers if you'll have a car

2) waiters/waitresses, bartenders (any one who serves food or drink)

3) dealers

4) bell boy if they do something for you, like carry your bags

5) The slot attendant, at least if you win something

That's generally about it. Don't worry about receptionists and people like that, and no need to go overboard.

EDIT:

6) Strippers