You can do it that way but i prefer to create the box size first and expand it.
I wouldnt say your way is simplier as its only a case of going to canvas size in photoshop and changing the sum from 480 to 960 once you are happy with your image. I just created myself a new one earlier and it only took me a few minutes. Dont see how it could be any simpler then that?
If you create it full size straight away you will have to still measure out 480 pixels and make sure its aligned to the horizontal centre. If you have text or something like that in the image that you dont want spilling over into the other screens just guessing would result in it not being totally inside that area and so you would end up going back and fourth to get it right.