The case that always p*****d me off when flying with guys from **** was the way they all, when cleared to a new level, always replied "Cleared 7000, leaving 8000." I would plead with them to wait until they had gone through the required ceremony of setting the new altitude, having it confirmed by the other pilot, selecting VNAV/FLChange and seeing the airplane actually start to move out of the old altitude before reporting leaving/left. I would say, "you are not actually leaving, are you? Just getting ready to do so, surely? And what happens if you select VNAV but the airplane is in ALT HOLD? Nothing will happen but you have already told ATC it is OK to let someone use our altitude, haven't you? (actually happened to me more than once, causing great grief to ATC and me!). The blank looks it earned me showed it was a waste of time trying to teach some of those yobbos what it is all about.."that's the way we have always done it!"
I had on old Airforce Wing Commander who insisted that we had to be at least 500 feet away from the old altitude, with an established rate of descent, before calling "left XXXX ft", and at the time I thought it was silly, but now I agree with it.
And never mind that in most parts of the world it is not necessary to call leaving. Clearances (such as Oceanic) will specify that you report "Reaching", and that makes more sense since separation will be based on the altitude you are currently occupying.
I guess when you get to see the belly of a 747 from real close you learn that little things can get you dead real fast.