This is really a loaded question. There's no simple answer and multiple strategies.
One strategy is the bleeding edge method. In that, you set the stage to scale NO_BORDER and exceed the stage in all directions with your artwork. That way it scales in aspect ratio to the device size but the images that bleed outside the edges of the stage will fill the black bar area. You then focus all your controls towards a designated 'safe area' for the device and assume the extra space may or may not be seen.
The harder but better method is NO_SCALE where you program like an Android device. You program your app so it detects the devices resolution and position all elements based on the available area. That means everything is constantly calculated based on available space, so it's a ton of extra coding, but it puts you in a very good port position for Android.
Bottom line, there is no magic bullet or Staples "Easy Button" to make your app work perfectly on all the (now) fragmented resolutions of iPhones/iPads.