The US dollar is easy for people to convert back into pesos but at the same time Canadian dollars or British Pounds are also easy for the staff to convert, there are exchange booths all over the place. Keep in mind that just like any other business the staff will pay a fee to exchange that money into pesos which they will use for day to day transactions. If you were to tip in pesos instead of dollars they have one less stop to make on their way home to get use out of the money. The days where the value of the peso fluctuated wildly are in the past so I wouldn't think that tipping in dollars is preferred as it once may have been. Just use the rule of thumb of tipping 10x as much in pesos as you would in dollars (10 pesos to 1 dollar equivalent) and I am sure they will be just as happy to recieve that!
