A cylinder exists in 3 buoyancy states, during the dive:
1) Negative
2) Neutral
3) Positive
All cylinders start negatively buoyant. When clipped, they'll hang.
As gas is consumed from the cylinder, the amount of negative buoyancy reduces.
For as long as the tank remains negative, no matter how slightly, it continues to hang down.
Aluminium tanks can become positively buoyant when sufficient gas is consumed from the tank.
Most steel tanks do not become positively buoyant, even if emptied. Invariably they will always hang. There are a small handful of exceptions.
Tanks that become positively buoyant will float. Any tank that's at all positively buoyant, even slightly, will float.
Tanks float up from the base. Buoyancy is not uniform across the cylinder. This is because the top of the cylinder has a heavy chrome/brass cylinder valve and heavy regulator 1st stage on it.
The base of the tank can (and for aluminium, usually does) becomes floaty, long before the entire tank becomes positively buoyant. This is because the weight of the valves/regulator etc at the top is supported.
Neutral buoyancy is momentary - the tank passes from positive to negative in the space of a single breath. For sidemount purposes we can pretty much disregard neutral buoyancy of tanks.
So... tanks are either hanging or floating. This factor dictates only two attachment points; one to the rear for when tanks are hanging and one to the front for when tanks are floating.
I'm not sure where this 'incremental' myth comes came from. Positive or negative buoyancy... hanging or floating tanks... take your pic.
Sliding D-rings are unnecessary from a cylinder trim perspective. There is no need, nor benefit, to "incrementally" moving them.
The reason to have sliding D-rings is if you find it more awkward to manipulate and operate bolt-snaps. Using a sliding D-ring removes the need to unclip, relocate and re-clip the cylinder bolt snaps.
For some divers, such as those in cold water wearing thicker gloves, that can he beneficial.
For divers without an impediment to their dexterity... and who are well trained, with attention to detail, in equipment operation.... there's little or no need for a sliding D-ring at all.