I'm with dextr0--spraying water, even in full sun, does NOT burn the leaves. While the droplets in clouds may help to make sunlight stronger and thus make us hairless apes more prone to burning (which is where I think this myth began) it simply cannot be extrapolated to a plant that requires as much full, direct sun as cannabis. I spray my little seedlings with water during the day, mid-day even, if I think it's too hot for instance. No problems.
There are products, as dex is alluding, that are phytotoxic, but those are other compounds, not just water.
As far as whether or not it's beneficial, I let the plants tell me. Some days it's really hot and they need to cool off and a little something to help with transpiration is just the ticket. Other days I just feel like it. Other days I don't. I wouldn't do it on cold days or cold conditions because there's nowhere for the water to go.
Moldy buds are caused for many reasons, if water is left sitting in the bud for days, then yes, it can likely cause rot (but that's not why, when or where I've experienced it).