Hi ByrdHouse,
It's a good question and the answer probably depends on tolerence for microbial build up. The reason that morning irrigation is generally preferred is that mold/fungus are less likely to build up because the sun drys up the surface wetness. In the case of a hot spell, the conditions likely to be extra dry and the chance of mold build up would generally be much less, so watering in the evening would provide more time for the water to soak in.
In the natural course of events, it rains both in the morning and the evening, and lawns and gardens continue to grow. So, in the case of a cycle that was programmed to only run on hot days it seems that there would be greater benefit to running it in the evening and giving the irrigation the chance to soak in all night. You could probably run it either way though.
Another point to consider is that, depending on where you live, the Yahoo high temp forecast might change during the day. The forecast temperature either might not be reached or the high temperature might exceed the forecast high. So depending on how your weather changes, you might get slightly better irrigation information by waiting until the evening of the hot day.
Jim