The only thing though is when using the Cleaner button CCleaner won't show what those cleaners will remove if it's something in the registry like for example a MRU/Recent Files list, it will only show physical files on the hard disk that it will remove.
You could use Resource Hacker (freeware) to look inside of CCleaner.exe (just don't save CCleaner.exe with Resource Hacker as that will remove the CCleaner signature) by expanding the INI section where the cleaners are stored to see what they'll remove - screenshot example:
Note that some cleaners don't reveal anything as some use internal code by Piriform which none of us regular forum members/volunteers know anything about.
There is a list (except as andavari said "specialkeys")
"c:\program files\ccleaner\cleaner.exe" /export
that said I don't see where the nk2 is cleaned
nk2 location is by default %appdata%\Microsoft\Outlook\
nowhere is that folder called by the default clean
In fact I went a step further and (on my office 2k7 machine) ran ccleaner as a clean with the exact default cleaning rules and after searched my computer for the nk2 file and it was still there, and still had the stored names. it also was not listed as to be removed upon computer restart.