Nigel,
I am with you on this request. Even if it is nothing more than a simple message right where the check boxes are for forwarding it might help.
If one is a person who gets a lot of mesages to them these are a major annoyance. That is messages that directed to someone else and have been copied to your In box. As someone else has said, "it only takes a 3 seconds" but multiply that by 20 or 30 and it adds up especially when you are pressed for time and only want to respond to your personal messages.
This is really a point of being discretionary with the forwarding. If you think a message will be of interest to others then check them off in the list otherwise don't. Many poeply will follow a thread out of choice anyway and they will see the message without it being directed to their personal messages.
I have also received messages forwarded to my box that were obviously not for me at all and I find it annoying at the least.