To generealize, my experience and observation has been that Mexicans don't tip well - and foreign tourists grossly over-tip. Though I do tip, I follow the patterns established by Mexicans who do.
Unless the restaurant is one of the top ones, I'll tip waitstaff about 10%, not more. In the coffee shops I'll leave extra coins (smaller demonination). I don't always tip, though - because - to generalize once again - I've found that service standards are so bad. I view tips as a gratuity for service above the level that should be provided and paid for in wages.
I don't tip taxi drivers because I find most to be out to cheat me (in so many of the destinations in the country). In Mexico City, though, I'll tip the person driving me to or from the airport - with the comment thanking them for a safe trip. Too many taxi-related crimes in that city and I want to reward the honest guys and gals.
Supermarkets treat the secondaria school kids like slaves and pay little or nothing, and expect customers to pick-up the wage payments. It's offensive but Mexicans accept the practice. I give the kids some coins, varying by amount of purchase - number of bags - but don't have a set amount. Sometimes I don't tip them at all - if they've only stuck a couple of small items in a bag. The kids earn a lot of money bagging and it goes to support their school supplies, and other purchases.
Relatively few tourists - myself included - rent cars or park them where someone will watch/wash them - and I have little experience with that. The people watching the cars are self-employed and they do provide a necessary service. You can bet on it being a certainty that if a car thief is going to take a car from a particular block it'll be one that the driver didn't ask the guy on the block to watch it. Consider it insurance. Inexpensive insurance.
When I use airport porters to help with luggage, which isn't too often, I give them 5 pesos per suitcase, but not more than 10 pesos in total.
It seems that everyone has their hand out, and most think we're all wealthy. We are wealthy by the standards of so many people in the country. But we don't have to be foolish when tipping. I don't tip out of a sense of guilt. I do it because I receive something in return. Keep focused on the local wages and the services being provided, and tip accordingly.