I have done both depending on whether my plant needs the water or not. If I find the soil dry in the morning, I'll try to get out there and water then. And if it is dry in the evening, I'll water at that time.

I think a big part of the issue of morning vs evening seems to actually revolve around overhead watering and wet leaves, where it's generally easier for the leaves to dry as the day progresses, which helps to reduce leaf diseases. Whereas watering overhead in the afternoon may result in leaves remaining wet overnight as the temperature drops and evaporation slows.

Of course there is the other school of thought that says that Mother Nature "waters" during the day and at night and plants do just fine, so what does it matter? Plus there is the issue of those who are in areas with very little humidity and adding moisture in the air via misting can often keep the plant's leaves from dessicating.