You own CS6. It will always work. I'm not sure if the CC install will leave it alone, but you should be able to re-install it later if needed.
If you want to start the CC deal it will cost $20.00 / month ($10 / month the first year if you sign up soon) for an individual PS subscription. If you decide to stop the service you can still go back to CS6. You will just loose any new features added after. Of course there is concern that camera raw updates won't be available down the road which could be an issue.
Unless they alter the file formats. In which case your CS6 won't open the files anymore.