One argument that was proposed by a non DIR cave instructor that made some sense was to clip the SPG to the left D-ring because:
1. At deep depths where gas use is high, it makes checking the SPG much easier with no need to work under the stage bottles. Regardless of the gas plan and the actual need to reference the SPG frequently or not, it makes it extremely simple to do as part of a visual scan and in that regard increases your SA.
2. In tight/low caves, the SPG when clipped to the left shoulder D-ring can be seen with no hands, there is again no need to work around the stage bottles and there is zero possibility of breaking trim and creating silt using this approach.
Prior to that I had carried it on the hip for at least 10 years, just because that is the way it was supposed to be done. But after considering what he said and trying it in cave/stage situations, it occurred to me that there are indeed circumstances where one standard approach may be a lot less than optimum.
You can practice 24/7 but the fact remains in a low, tight silty cave with one or more stages, checking the SPG without silting or contacting the cave with yourself or a bottle is demanding, a bit of a PITA and one that can be easily avoided by just clipping the SPG to the left shoulder D-ring before things get snug. The "equipment solution for a skills deficiency" argument can be used, but I had to ask myself when just being smart and planning ahead begins to make a lot more sense.
Personally, when the situation dictates, I clip it to the left D-ring and in all other situations I clip it to the hip D-ring. I am sure the use of two procedures could someday strangle me in my sleep, but its a chance I'm willing to take.
If there is a valid argument against clipping to the left shoulder D-ring, it would be the potential to confuse the backgas SPG with an SPG on one of the stages or deco bottles. But different/smaller SPG's on the stages and/or turning the stage SPGs around to face away from you when not in use reduces that potential. Besides, you can see that the baclkgas SPG has a clip on it and that it is clipped to the D-ring, so you'd have to be a moron to get it confused.