I uninstalled CS5 on one machine to put CS6 on it. I want to put CS5 on another machine but I didn't do the unlicense thing before I uninstalled. Do I really need to do that or Is this going to be OK? Or is there something else I can do besides uninstalling 6 and reinstalling 5 just to release the license?
Yes, the only way I know to deactivate it is to do so in Help menu of PS. There is no need to uninstall old version unless you absolutely need the hd space.
So you are saying I can just reinstal 5 without unistalling 6 then I can go to the help menu on 5 deactivate, then unistall 5 and 6 will be OK?
Not necessarily. Some experience file association and other file problems when uninstalling pervious versions. They should be installed in the order of date. So ideally you should uninstall CS6, install CS5, deactivate CS5, uninstall CS5, install CS6.
You can run 2 activated copies of each version without problems. Personaly I prefer Bridge CS5 to Bridge CS6, so that is the way I run them when don't need extras of Bridge CS6..