You have to remember that the Mint updater by default doesn't update packages that cary a 4 or 5 level that could potentially break your system, this would include packages like dbus, the kernel, xorg and linux-base, as such it handles reboot notifications using it's own mechanism. Since Mint update does use it's own mechanism and not those other files, and the fact that your updateing outside of Mint update, might be the reason behind your issue. If CD is already giving you this notification I see no reason to install kubuntu-notification-helper at this time, but if you were to quit using CD at some point and still wish to get that notification, then I see no reason why this package would cause you any issues.
PS: Please keep in mind that Mint updater is the prefered method for handling updates for the reasons of system stability that I already mentioned previously in this post.
PS: Please keep in mind that Mint updater is the prefered method for handling updates for the reasons of system stability that I already mentioned previously in this post.