I need to free up disk space on my MacBook Pro. Is it okay to delete LR5 once you've become a subscriber to Creative Cloud?
If you are certain you won't be needing Lightroom 5 anymore, then it is safe to remove it. There is no interdependence between different versions or even different updates of Lightroom. Every new installation of Lightroom, upgrade or update, is a complete new installation of the program.
One thing you might want to check is if all your settings and User Presets have been moved, copied, over to LR CC.
But removing LR isn't going to free a whole lot of space. The actual program only takes up about 1+GBs of space.
This is on Windows but it would be similar on OS X.