To not be rude in return for good service, drop 15%. If you spontaneously decide to go to a restaurant that requires a reservation, and you expect to be seated, you need to grease some palms. I am fortunate in that I can afford to treat people well and I do. The other point to consider is, are the tips shared and does the owner or manager get a cut? Regardless, it's always a good idea; especially abroad, to treat others well. The proverbial Golden Rule is a good one to follow anywhere on the planet. Good luck.