To clarify my point - CCleaner is not intended to clean up anything except for the files it can target, so there is just as much chance of your old sensitive data being at the end of foo.tmp as it being at the end of notepad.exe - and in the latter case, CC wouldn't clean it, so why be so concerned about it on the few hundred files that get removed by CC?
Buy PGP - for about $30 (I think - personal desktop?) you can replace all "delete" actions, by your or by an app, with secure wipes. Then you can disable CC's secure wipe (no point doing it twice).
Or just get a free eraser tool, and wipe free space with that, including slack.
Reiterating - "sensitive data X" is statistically unlikely to be in the slack space within a few dozen MB that CC looks for, compared to the size of your drive.
However I would like to see CC (or Windows, or every app for that matter) address user concerns, even those I disagree with

- I am beginning to wonder why, in these days of very fast disk writing, we have not seen app or OS creators start to blank out disk space up to the end of the last occupied cluster...