I tend to throw in photographed skies in post. Or in the backdrop. The different between a real sky and the Carrara look is large (which Bryce in it's 'volumetric world' processor bashing mode does well, and Vue looks like it does a good job). If the sky had any significance in the picture I think there is much more to clouds then just a layer of white-ish stuff. There are a lot of free photographs on the net that will do the job easier and better then Carrara at this moment. Maybe it feels like cheating to get it from 'outside', but, especially with the broad spectrum of material available, why not be practical sometimes? When Carrara 4 or 5 does volumetric clouds, and my mac can render them in one night I will reconsider of course.