Assuming we are creating Assets to be used on iOS in order to create textures that are then mapped to the surface of Quads in 3D Space, is below recommendation and its reasoning appropriate?
Try to avoid unnecessary* transparent pixels whilst ensuring a Power of 2 texture size.
Since the texture will occupy the same amount of RAM whether it has its content spread across its entire surface or not, try to use as much of the available surface for the content as possible. This will improve visual quality when downsizing the texture (e.g. to generate Mipmaps).
*e.g.: a padding of transparent pixels around an image could be considered unnecessary if the texture will be resized anyway.