This really depends on how you design your game.
Also, it depends from how you resize the game, most people use Letterbox scale, I used it too but with experience, I've found that Scale outer works better, and publishers prefer it.
Letterbox scale tries to fit on your screen and leaves black boxes to fill remaining area.
Scale outer tries to fit on your screen and shows more layout instead of black boxes.
I've prepared some comparison for you:
I created a new project, set the background colour to be red, and filled the screen with one, cyan sprite.
The window size is set to be the resolution of iPhone 5.
Check how this looks with using different scale modes:
Scale outer -
You can't overcome the black boxes in letterbox, however you can fill the layout area outside the viewport with some background and no red area will be shown.
Read about aspect ratios, if you create a game for 320x480 it will work perfectly on any resolution which is a multiplication of those values. So if you create a game in iPhone resolution (320x480) it will work well on iPhone 4 because it has a resolution of 640x960, divide those by 2 and you'll have 320x480 resolution.
So, you can use iPhone5 resolution to scale it down, but then you will need to pick one of the scale modes and act accordingly to it's consequences.