If I remember rightly, Adobe say that you shouldn't have multiple copies on one Mac. I can't remember where I read that, it was some time ago. Having said that, I've always ignored their advice. I have CS5, CS6 and CC installed on my Mac and all run fine.
The only issues I've had is running AppleScripts - sometimes the script will launch the wrong app. I think that's more a case of the script assuming only one copy of Illustrator is installed though.