Represent your home town or country right by tipping in accordance with our local norms. The waitstaff average is 18-20% in Boston and surrounding areas. 15% might be acceptable for only the very worst service but for normal service 18-20% is the norm. 25-30% is common for good-to-great service. This may seem extreme to some visitors but servers make a very low hourly wage (currently $2.75/hour.) They depend on appropriate tips to make a living wage. Sometimes when we travel, we are stingy with our money because vacationing is expensive and we'll never return to these restaurants again. But servers cannot explain this to their landlords and gas stations and grocery stores when it comes time to pay their bill.
Even if Boston tipping etiquette in restaurants seems excessive to you, please represent your fellow visitors well by tipping properly in Boston and wherever you go. And remember that tipping norms may change over time. Years ago, 15% was acceptable in MA. With inflation of milk and bread and real estate and gas, this is no longer true. As of 2009, 18-20% is the standard tip for service in MA.
Tipping is more or less compulsory. In restaurants, waiters expect around 10% to 20%, as do taxi drivers, while porters and bellboys anticipate around USD 1.00 or USD 2.00 per piece of luggage. You are not expected to tip in fast-food restaurants or cinemas.