Portrait upside down
iPhone 6 Plus introduces an iPad-like orientation setting—home screen can be used in all 4 orientations, including portrait upside-down. I think the app developer may consider adding the iPad orientation rules in their iPhone app. The rule is that any app have to at least support 2 orientations, particularly for portrait app, they need to support portrait upside-down because this is just as nature as holding iPad without worrying where the home button.
One more benefit is that your user can then use your app while charging their iPhone. For example, I like to keep my iPhone screen on to show my today’s task list. I need to plug the cable and place the iPhone upside-down on the stand.