I went climbing in Mexico in mid May, 2007 and had only a little trouble with weather. While the US winter is the best time to climb the volcanoes because of Mexico's dry and stable weather patterns that time of year, May was fine. Overnight thunderstorms a couple of nights, but they cleared out before we needed to be on the trail. And chance of afternoon lightning and rain. I would say, if you're used to climbing in places like Colorado in the summer, then you'll be fine in Mexico. You just need to be more aware of what the weather is doing than you would during the typical climbing season.