recently i had tried to make task-bar invisible in maximize mode but unfortunately nothing worked.
i had some research over MSDN and stack overflow and most of them said make window border equal to none and maximize window.but it didn't worked.
BTW: i am using Metro-window and it has a hide task-bar functionality by itself.but it didn't worked either.
My experience on Windows 8.1 is that setting WindowStyle="None" WindowState="Maximized"
(as most people say) is not enough.
To hide taskbar I had to set ResizeMode="NoResize"