No, it's not necessary to have them to run the most recent Photoshop.
Keep in mind that if you uninstall, you can only reinstall the current version and the one previous. It's usually a good idea to keep one older stable version when a new one is first released.
Many folks say that if you uninstall, do it in reverse order, then reinstall the newest in forward order. Be sure to back up assets such as actions first. As for me, I never uninstall, so I have a number of versions.