I don't even leave cookies and cache on my PC, so why would I leave those files on my BlackBerry, removing cookies and cache files especially if your a heavy internet user via your device can be a huge memory saver.
To delete these files simply open your Blackberry browser, then push the Menu key and then follow this sequence: Options > Cache Operations and finally choose "Clear History & Clear Cookies."
This server two great purposes, the first is the memory saved, second is the privacy issue, your cookies are now deleted and you are once again surfing semi-anonymously.