In cheaper restaurants it's not necessary to leave more than a few coins in the change plate. In more expensive restaurants, tipping is customary. Even if a 10-15% service charge is added to your bill, you're expected to give around 5% to the waiter directly and perhaps the same amount to the maitre d'. Porters expect a dollar or so; in taxis you might like to round up the bill; in other situations, for example, helpful guardians at archaeological sites, delicacy is required. Although a tip may be initially refused through politeness, you should offer the money a second and third time. After three refusals, you can safely assume they really don't want the money. In hamams you should tip 10% to 20% of the admission price to the masseuse/masseur.