Talk about beating a dead horse to death........
1....Some Mexicans tip, some don´t
2....Some foreigners tip, some don´t
Do, and how much, or don´t, is a personal choice, not a law, not a standard. Do whatever grabs you and quit worrying about how many Mexicans tip or don´t tip, as anything you do or don´t do is going to change the price of the gasolina.