My view is that various cloud services have virtually nothing in common, so having a tag to group them all serves no purpose. It's better to use specific tags like dropbox, for example.
Izzy thought some combinations were useful, like cloudprinting or cloudbackup. I think those combinations are potentially useful categories, but would be better represented by a single tag like cloud-printing. There's not really a use case for filtering by cloud, since printing and backup have in common; you wouldn't even use the same cloud service for both.
What say ye?