Lot's of different answers on this one so I can only give you my opinion and what I do.
There's no point watering plants in the midday sun. The water will evaporate before it gets as far down as the plant roots.
Early evening is best. The plants will have enjoyed the benefits of the suns heat and will appreciate a good drink.I've removed a link on this post. It's really good advice, but is copied from a Yahoo answers website, where the original was posted three years ago. So it's one of those fake fakes, I'm afraid. Cheers, M