[opinion]Not to quibble but I find that programs that don't need to be run for long times with no user interaction should not include close=minimze. Windows provides us with a minimize button.
Minimizing to try makes sense for Instant Messengers, and I could even (with some stretching) understand doing it for defraggler. But ccleaner is cleaning for (at maximum and not running WFS) 30 sec-1.5 Min.
If you really want it to just run in the tray just use "ccleaner.exe /auto"[/opinion]
The Developers read all threads, though rarely comment; they do take each and every, viable, suggestion into consideration. So, you may indeed see a Minimize to tray option in future builds of ccleaner.
Pressing the Close Button won't Minimize CCleaner to the Tray, but my version does have an option for Minimizing to the Tray using the Minimize Button.