People argue about this, but if you look up expert sources (gardening websites, magazines, gardening TV programmes etc) you get the same answer: it's best to water in the early morning. This is because the plant will then have all day in which to absorb the water and create chlorophyll from the sun. If you water in the evening the wet plant will spend the night in colder air, and this is the way you can get mildew and similar damp-related problems; the plant can also be more vulnerable to diseases in these conditions.

As well as watering in the early morning, it's also advisable to water quite heavily and not too often (though of course this will vary a lot according to the type of plant and the climate it is in.) But the general principle is that heavy, infrequent watering will imitate the natural process of rainfall, rather than a light frequent sprinkling.

You can find more good general advice here.