The tip, also known as “propina” is a standard part of the Mexican culture. Ten percent is standard at restaurants, though fifteen percent is more common in American-oriented areas like Cancun.
All the time take a look at the bill first, before you pay. The service (servicio) is sometimes included mostly at more formal places (usually 10-15%). In this case you are not expected to leave anything additional, even though it is very appreciated.
In luxury resorts, the tip is M$10 or M$15 per bag, M$10 or M$20 to parking attendants, M$20 – M$30 per day in the room.
Staff at smaller, Mexican-run hotels don’t generally expect tips, unless the clients are most foreign. At the petrol stations, the tip is M$5 – M$10.
The restroom tip is a standard here, M$5. Sometimes a sign is posted, sometimes not.
Cancun taxi tips: usually the taxi drivers expect only the flat fare, but if he helps you with the luggage you could tip him with M$10 or M$20.
Remember, M$1 is the mexican dollar.