It's about custom in the location. In the USA, a tip of 15-20% is pretty common. At a high-end restaurant, 20% on the total bill (food + liquor) is normal and a bit more for excellent service. Remember, that tip is being split between the captain, the waiter, the wine steward and the bus boy (or more).

In other countries, it's built into the cost and yet in others, it's added on as "service." Some places in the USA, require it on large parties.

While agressive tactics aren't fun, it's also true that there are people who don't frequent expensive restaurants and think that if they run up a $500 tab for 3 people, it's OK to leave a $50 tip because that's what they leave at a local place back home where the waiter is handling 7 tables.

You can always be cheap at a place you're never going back to (doesn't make it right), but don't expect a level of service or not to have someone spit in your food in the kitchen, when you go back !