Before the sun is about to hit or just as the sun is hitting in the early morning.

If that doesn't work then water early in the evening but allow time for the sun to dry the foilage.

Thats my take for any plants.

Honestly, I water when I can and have never had an issue. I watered this morning because they were flagging badly and it was only 10 am. If we had just had a good amount of rain and they were flagging I would assume it was just from the heat (they can be prima donnas) and would wait to see how they were at night. If they were still flagging after work (say, 6 pm) I would water then.