Thanks for the replies! Much appreciated by this new member!

So the bottom line is that is just something some airlines may require for their own convenience, although not all of them do, hence my staff office just stressing its importance all the time so to make sure I get all the chances, regardless the details of the ZED agreement?

I agree listing shouldn't be a big deal on condition you can do it online or via one simple call, but unfortunately I can tell you stories about having to call about 20 times before I got the right person on the phone or having to call in to international numbers during peak time (because the airline didn't have an office in my country)...
Those were the cases which made me think about the necessity to follow the listing procedure, since I knew for sure there were still 40 open seats on the flight, hence the question!

About the listing itself:
Do you generally call in for it or do you use dedicated websites?
Or even better: does your staff travel office do it for you?
At my airline, they take care of the listing if I fly on my own airline, but for all the others, they leave it up to us.
For that, we have a list of phone numbers of all the airlines we have ZED agreements with and other than the well-known flyzed.com list, we always have to call in, although -thanks to this site- I've learnt that AF for instance has a nice website where you can get yourself listed online! I've managed to 'crack' the login and password for my airline, which isn't very difficult actually and got myself listed very easily for a flight! How convenient, especially when you want to push back your return date during your trip!

Does anybody know of other airline specific listing sites which are more or less open to all?
I've read BA has such a site too (and even without a password) for instance, but I don't seem able to find it...