I usually tip $20-$100 MXN depending on the level of service.

If I'm near the US/Mexico border then I may use USD, usually $1-$5 or use a mix of USD and pesos since the casa de cambios can be easily found. I apologize if I do the latter.
For cities outside the US/Mexico border, I only use pesos.

I avoid using USD for routine transactions because of the unfavorable exchange rates/pricing and extra fees, but I will use USD to convert to pesos as needed.

Mainly though, I try to have pesos on me, cash and on my Mexican debit card regardless of where I am in Mexico. Mexico is mainly a cash (in MXN) based economy so showing up when I only have USD, US and Mexican debit cards is going to be awkward.