Thank you !
Well, apparently it would always be saving fuel puting CI=0 as soon as you have a lot of tail wind ... but how much ?
At least, enough to not affect the normal flight time (lower speed but tail wind) especially when there is no need to arrive early at destination as the next departure time is not going to change anyway.
I'm thinking about this from a couple of days, and I might suggest my airline to change our fuel policy about this thread.