If also plan to use kdesu, then add the lines below to your /usr/share/kubuntu-default-settings/kde-profile/default/share/config/kdeglobals or ~/.kde/share/config/kdeglobals.
Code:
[super-user-command]
super-user-command=su
When I installed Kubuntu for a friend, I even went as far as disabling the admin groups privileges to run all commands using sudo.