If you don't mind me adding my 2 ¢ents here, one good example of where I think you can use "return false;" effectively is to kill the context menu (a.k.a. right-click) of a page when you don't want the users playing around that much.
/me braces for Bear's corrections. ;)
"I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use."
-- Galileo Galilei
Sorry but disabling the context menu is a silly thing to do. What purpose could it possibly really serve? Anything that can be done via the context menu can be done by other means that cannot be blocked. So other than to annoy your users, what's the reason?