In theory. In practice, you could get away with supporting just iPhone and iPad layouts, or one or the other if not going Universal. (And Autolayout helps you move forward when you're ready pretty easily.) Also, you don't have to worry about things like graphics hardware differences, poorly-implemented APIs, and OS versions nearly as much.

Is it even possible to tell the difference between the iPad and iPad Mini in software? When it first came out, it wasn't, on purpose.