I'm usually good for 5 pesos or so just for gas if the attendant is courteous and efficient. If I ask him to add air to the tires, check the oil or clean the windshield I kick in an extra 5 pesos.

Unique Mexican tipping situations that visitors may not be aware of:
-- Bag boy or girl at supermarket (2 to 4 pesos - these kids are hard workers; increase tip when I have a lot of groceries to bag).
-- The guy in the parking lot that "helps" you back out of your space by watching for traffic and guiding you (1 or 2 pesos).
-- Car watchers (15 pesos/hour if he is actively watching my car).