I stayed at a Mexican AI resort earlier this year, and we tipped in both pesos and US (took pesos, but tipped a little more than expected so ended up using US too) - they didn't seem to mind too much which, but as I understand it pesos are easier for them to use as there are now restrictions on US cash. I only tipped for good service, I just point blank refuse to tip someone who is rude, inattentive, brings the wrong things etc - fortunately there were not many. At an AI it is pretty hard to tip a percentage, as the meals are all included, so you have no idea what the actual cost would be (i.e. that meal in a mexican restaurant a few miles down the road, is going to be very differently priced from the same meal in the US - so which do you tip based on?) We just tipped what we felt comfortable doing - some people tip more, some people tip less, some not at all (in fact, our travel package literature specifically said the cost 'includes all tips and gratuities')