Autologin as root is so insanely insecure that most likely no login manager will allow it under any circumstances, so you might as well let that go right now
If you are using GDM (the GNOME login manager), however, you can use the GDM Configurator (look for it in your menu) to allow root to log in manually at the login screen. However make sure to uncheck
"allow root to log in remotely via XDMCP", unless you expect to have to log in to your PC from a remote PC and you know how to secure your computer against unauthorized access.
If you are using KDM (the KDE login manager), you can also allow root to login manually by unhiding the root user in the user browser. This can be done in the KDE Control Center. I don't use KDE that much, so I don't know the steps offhand, but it's pretty easy to figure out once you're looking at it in the KDE Control Center.
Hope this helps.