Thanks everyone.
The interpretation seems to vary around the world. I asked the same question to some UK ATC guys and they came up with various different answers:-)
Basically, most of them agree that they want you to stay on the extended centerline; don't stray.
I fly based in the M.E. and in this part of the world only God knows what the interpretation is, because they really don't care if you are flying heading or track. ATC wouldn't know it anyway:-) They see you tracking the centerline and seem to be a happy bunch about it, regardless of where the nose of your airplane is pointing to.
Recently, I had a little row with a check pilot on this matter so I went around asking questions:-))
In the B777, we usually select track for take off, because LNAV will do the job anyway, regardless of what is in there on the MCP. The doubt appears when you don't have a SID or ATC tells you to fly runway heading.
Thanks again