Quitting the application Calculator saves all the preferences like basic/advanced mode, precision, show paper tape, recent conversion, etc. However, clicking the red close button doesn't save these settings. Is it a bug or a feature?
Updated exchange rates are not affected (they're saved into a different file).
But if you want your settings to stick, make sure you quit with Command-Q, not the red close button.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040728054606402