In normal circumstances that process should die when the executing program terminates.
However, there are some programs that do not clean up well and NVidia's control ICON, in the systray, also keeps it alive.
Most of the time, the rundll32.exe program is running some spyware/malware DLL file.
If you have done a decent job of cleaning up unneccessary programs from running in the background, then this process will not appear. Generally speaking, the appearance of the 'rundll32.exe' file in your process list could be an indicator of spyware/malware running on your computer. It is not always the case, but it is a good indicator.