I wrote an animated guide to do proper asset scaling and multi-resolution in Cocos2d-x. I am hoping it to be very clear on how to approach the problem, and how to solve it. After the post, a Cocos2d-x user reminded me this is one of the most discussed topics on the forum. So I decided I should post it here.
We, as 6x13 Games, released 4 mobile titles using the described method, and I know many others use similar approach. So, do not hesitate to use it in production. Good luck!