Hi there!
When I took the SDI Solo Diver course last year, my instructor recommended me not to place the BCD carrying handle over the cylinder valve, so that I could use the handle to carry the BCD. His recommendation sounded perfectly logic to me, as I suppose that that is why the thing in question is called a "carrying handle". Right?
This year I joined the local dive club, which is affiliated with CMAS. In the club I noted that everyone there has been taught to position the handle over the valve; I've got the impression that they do it simply because it's the customary practice in the club, without asking themselves what the justification for the practice is.
It seems to me that an advantage of not placing the handle over the valve is that the handle can be used as such until the scuba equipment gets disassembled, which may be useful, for example, when boat diving. By leaving the handle free, you can hand an assembled equipment to someone in the boot, by holding the BCD and the rest of the equipment from the handle. On the other hand, I wonder whether leaving the handle free would not transform it into a potential source of entanglement in certain sites.
I was just curious to know the reasons why you position or not the BCD carrying handle over the cylinder valve.