Quote - "negative values in the atmo editor can yield to grainy clouds"

Hmm... I always use negative values and I don't normally get grainy clouds. In my experience, grainy clouds are normally the result of too low AA. But I tend to use the same kind of clouds over and over, so maybe I've yet to come across clouds where this happens due the quality slider.

Quote - "splotches if you have a strong sun glow and strong scattering anisotropy"

Yes, and also in shadow areas when the light balance is a lot towards "ambience". In my tests, I found that the more the light balance is towards ambience, the less negative can the quality slider be, or splotches appear in the shadow areas. If the light balance is, let's say, at 90%, then normally the quality slider can be set at -2 or even less. But if the light balance is so high, shadow areas tend to be unnaturally dark, which can be compensated by increasing the sky dome lighting gain.