You should pay in pesos, across the board in almost every situation. The only convincing point I have ever heard in favor of dollars is for one very specific situation: tipping strippers. But that was when the exchange rate was less favorable against the dollar. In that case, you may have wanted to bring a small stack of $1 bills (or a large stack—I have no idea how much/whether you love the strippers!). But now that 20 pesos equals almost exactly $1 USD, you can simply make sure you have a stack of 20 peso notes if you are visiting a strip club.
Paying in cash is preferable for all small transactions. The exchange rates are very high at the airport, and the fees at the airport ATMs are also quite high. I don’t like to waste money, so I prefer to get pesos at an ATM in town. For those who are staying in Zona Romantica (as all gay tourists should—see my advice on where to stay in Puerto Vallarta), the two most convenient ATMs that dispense pesos (be aware that some ATMs nearby only dispense dollars, so be sure to check the signage and messages carefully) are located at the Banorte at Olas Altas 246, one block south of the park (Parque Lazaro Cardenas), or the Intercam Banco at Ignacio L. Vallarta 130, which is the first block of Zona Romantica after you take the bridge over Rio Cuale from Centro Historico Puerto Vallarta. The Banorte ATM in particular can be quite busy, with a line forming on the sidewalk.
To use the ATM, you can first choose to use English or Spanish for your transaction. It will guide you through several steps, and at some point it will offer you the choice to accept an exchange rate set by the bank. DECLINE this exchange rate—many people accept thinking they must accept in order to continue, but that is not true. If you decline, you will get the daily best exchange rate rather than the somewhat inflated exchange rate they offer. My choice is to take out a large amount of money and then keep the bulk of it in the safe in my hotel, while carrying only enough for the day I have planned. I do this for convenience (fewer trips to the ATM) and also because the ATMs OFTEN RUN OUT OF MONEY, especially on very busy holidays or during events or celebrations. I have even found both of the usual ATMs out of money, in which case I walk over the bridge into Centro and visit the large Banco Azteca located one block north of the bridge.
At restaurants that cater to tourists or that are somewhat more expensive, you can usually use your credit card. A smart thing to do is to make sure your credit card does not charge a fee for international usage. And don’t forget to notify your credit card issuer that you will be traveling! There are few things less fun than being stuck in a foreign country with no way to pay for things!
So why should you pay in pesos? Out of respect for the local business people you are dealing with, and for your own benefit. It is an inconvenience (and an expense) for locals to convert dollars to pesos—they pay an exchange rate just like you do. Many restaurants will accept payment in dollars, but they set their own exchange rate, and across the board it will always be to their benefit, meaning they charge you extra for the inconvenience and expense they will incur when they in turn have to convert those dollars to pesos. It benefits everyone if you simply use pesos. And, a sidenote: there is a persistent myth that locals prefer to receive tips in the form of dollars. This is simply not true. Tip your waiters (and masseurs and taxi drivers and anyone else you may be tipping) with pesos.
My method for converting prices from pesos to dollars, so that I can better judge the expense of an item is to drop the last digit (rounding up or down to the zero or the next increment of 10), and then divide that result in half. This may require tweaking if you want an EXACT calculation, but for a quick-and-dirty estimate, it is useful. $100 pesos becomes 10 when the last digit is dropped, and divide 10 in half to get 5. So, 100 pesos is 5 USD. 750 pesosà75à$37.50. If it helps. You can also think of one peso as one US nickel. Ten pesos is 50 cents.
I try to judge my spending and my withdrawals to avoid having too many pesos left at the end of my trip. If I have more than I need to pay for the taxi to the airport, I prefer to exchange the pesos back to dollars in Zona Romantica, because the process at the airport is time consuming, tedious, and expensive—they charge something like a 13% fee. The place I have used is CIBanco at Olas Altas 513, just across from The Palm Cabaret and Bar, right in the heart of the gay area. They have always provided me with quick and convenient service.