I think I agree with Matt in that the additional AO gives it just a bit more reality, but it also depends on the species of plants how dark shadows should be. Some leaves let more light through than others, more reflective leaves will have more reflected light in the shadows as well.
I tend to up GI to get lighter shadows on the whole, and also decrease opacity of the leaves to get less dark shadows. Adding more haze or more cirrus also helps in getting a tad more light in shadows, so I use that as well.