Using Force Quit

Completely harmless.

This is not correct. Force Quiting applications can lead to data corruption due to where the program is unable to write back data in cache back to file. The more complex applications that process media, like photoshop, Aperture, iPhoto, that handle large files and databasees would be more susceptible than something simple like text edit.

Force Quit is intended to be used as a 'survival tool' to prevent having to do a hard shut down. It is located under the  menu as a convenient location.

Quiting your applications gracefully, is the desirable way of shutting down. Command + Q is pretty fast.

roam's comment encouraged me to do additional research on Force Quit. The official Apple line follows:

http://support.apple.com/kb/HT3411

In other words, Command-Q is the preferred method of quitting processes. But with processes that won't quit - and that could be anything from Mail to Safari - Force Quit remains a standby.

Instead of habitually relying on Force Quit, regardness of need, simply look at the items on your Dock. If your dock is set up horizonally (the default), Finder and any active items will be colored more brightly on the lower edge; that's a sign to quit active items, should you choose to do so. If your Dock is set up vertically (as is mine), active items will denoted by a white tick on the outer edge. Finder is always on, so that's always ticked.

I don't know if Force Quit will cause any harm, but you can avoid it entirely by looking at your Dock before you shut down.