I'm actually finding the reverse, for a still image, your right, low sample rate is fine and even introduces some useful noise.
But for an animated sequence that noise is, er, noisy and needs to be removed so the sampling is set high: quality=2, samples=312, but the cloud depth is 3000m and a radius of 50m (which accounts for the high sample rate).
I'm up to 11 cloud layers for the waterfall, including the foam at the bottom. It's starting to look quite believable I think. Quite fiddly though.
Richard