Another way to combine those points: Consistency is good, but its value degrades gracefully. Especially in a simple app, you can mess it up a fair bit and users will tolerate it provided they can still manage the result. But following the local human interface guidelines does make the app more usable, and the more complex your app is the more that matters. Unless, of course, your application is such a big deal that it can set its own standards and users will learn it unto itself (EMACS and Photoshop come to mind); that isn't free, but as long as you're internally consistent it at least just comes to up front cost.