Unless you think (as I actually do) that you will need the older versions, I would uninstall everything, clean out the registry, and do a defrag first, then install the latest version.
Personally I still use the older versions for older projects because I find I have a lot of hassles with plugins and such when opening a project in a different version than the one it was created in. Especially when the difference includes 32 vs. 64 bit programs.
Also, I have found that installing the new version beside the older ones can sometimes very subtly change how the older ones look and operate. Like changing a "0" value to "0.0". And if I notice these visual changes I wonder what might be changed behind the scenes.
Steve