BUT to answer the question of the OP, it matters little. Most folks keep the arm on the centerline...this is to teach the body to get used to the inner elbow position and to generate the forward intent while on the inside attacking line. So I agree with Kevin to this point.
However I also agree that it is an action, as Terence points out. It is meant to serve as a controlling hand. This control could be simply controlling space, retaining the inside line with punching, or it could be controlling in that you've used it to stick to the attacking arm and redirect it around to gain an inside line (fook to huen). I've known people who think of man-geng-sao as a derivative of the fook sao.
There are probably a NUMBER of reasons why things are done the way they are done, to build attributes, to build application skill, to build muscle memory, etc etc.
The only right and wrong is based on the end result...does your fook sao work for you? Great! Because everyone is built differently, everyone will have a slightly different variation in their fook more than likely.
"I don't know if anyone is known with the art of "sitting on your couch" here, but in my eyes it is also to be a martial art.
It is the art of avoiding dangerous situations. It helps you to avoid a dangerous situation by not actually being there. So lets say there is a dangerous situation going on somewhere other than your couch. You are safely seated on your couch so you have in a nutshell "difused" the situation."