NEVER!
I find Cover Flow annoying at best. If you can't find your documents in list format you should learn better file management. I wish there was a way to disable it for the iPhone as well as going into landscape mode with SMS and Email.
there is. on my jailbroken iphone i install 'rotation inhibitor' which lets you toggle rotation on and off any time you want via sbsettings. by far the best and most useful thing to ever come to the iphone.