In Mexico, waiters, maids in hotels, porters and bag boys/girls rely heavily on tips as their wages are very low – not unlike north of the border.
Taxis quote you the fare and it isn’t necessary to tip additional unless they handle your baggage-- $1 USD per bag is appropriate-- for you or wait while you are shopping, etc. Typically it is the same in the airport or the hotel – $1 USD per bag. It is also customary to tip the chambermaid in the hotel US$1 per night.
Tips to waiters in restaurants are similar to back home – 10% to 15% depending on the service. However, if you find yourself in a small very basic restaurant, don’t be surprised if the bill is extremely cheap; in which case you might want to up the tip a little – after all the service is the same.
The bag boys/girls at grocery stores who pack your groceries typically earn no salary and rely entirely on tips. Two to ten pesos (20 cents to a dollar U.S.) would be appropriate depending on the number of bags and whether they carry your items to the car for you.
Gas attendants aren’t normally tipped unless they do some additional service like window washing, air in tires, etc. At just about every stop light in major cities, someone will try to wash your windshield. If you don’t want it washed, be quick. Say “no gracias” (firmly but politely) and wag your finger back and forth in a “no” fashion. Sometimes they will still go ahead and start washing the windshield. You can repeat your “no” and hope they stop or you can let them continue and pay them. We tend to do a little of each, depending on our mood at the time and whether or not the windshield is dirty or has just been cleaned at the last light. When you do tip, the appropriate amount is about two to five pesos.