Seriously. Nobody knew this?
I'll go a step further: You're no longer exchanging a view, you are tacitly indicating that the correspondance is now one way: "me (and the boss) to you". Cc'ing any senior means you are at best a 'toady' sucking up to the senior or a 'bully' using the senior's e-presence to quell any discussion.
The action also has side effects. It can clog a senior's inbox losing useful correspondance in the 'noise' of all the frivolous Cc's.