we have never used pecos, unless we get change back from a purchase we make. Than we use those before we go home.
If you're going to Cancun, why would you exchange it at all? We've not been anywhere in the Yucatan where they don't accept USD. They might have to give you back pesos in change, so you'll want to know the rate, but they'll accept the cash, don't worry about that.
My thing is that the employees LIVE in Mexico so they have to pay their bills in pesos, therefore they need to change out the money. So now our $1.60 tip drops to what, like barely over $1? So I guess if you are accounting for that etc., then you are all good. As a former server and as a "housewife" that substitute teaches and never knows if I may have to be a server again someday, I'm very concerned about karma so we plan to eat the conversion fee on our end and tip only in pesos. Again, like all my posts, not preaching - just my personal opinion (Since CCC has picked up so many new members, I have to remember I'm not the newbie anymore -weird!!! - and newbies don't know me yet ).