[David Honan]"So am i right in thinking clearing the Ae version CC cache once in a while is a good idea? Is there a trade off?"
There's no need to do this for housekeeping's sake; Ae will automatically prune the cache when it needs space, and the cache sitting on the disk won't negatively affect performance.
If you do empty the disk cache, you lose the advantage of the disk cache which persists across sessions. Your renders and RAM previews may slow down if Ae has to re-render things it had already previously rendered and cached (but you deleted).
The only time I empty my disk cache is if there's some kind of problematic frame that's been incorrectly cached (like a text layer with a missing font).
[David Honan]"New to Ae I'm beginning to learn preview / rendering etc thumps your RAM and some housekeeping is needed once in a while to keeps things smooth."
What kinds of problems are you running into? What version of Ae?
I notice that you have 8 GB of RAM -- that's not all that much for After Effects and you would certainly benefit from an upgrade.