I qualified the statement with technical backmount training because I personally split bottles between left and right when diving OC sidemount, and when diving with my sidemount rebreather, all of the bottles are on the right because the rebreather is on the left and it makes reaching things quite annoying and this holds true when diving with doubles as bailuent/dilout or with a sidemount bottle on the right.
It's something I'm still putting a lot of thought into though. I want the breather on the left because I do a lot of diving with it with backmount doubles and need the long hose to be free to deploy but also to keep the right hip free for can lights. This basically means the left side of my body is completely full and I can't put anything else over there. A set of LP120 doubles will get you a LONG way back in a cave, and by the time that I have to bring stages, I'm going over an hour from the door in the 4ata caves and at that point I'll just fill stages with 30/30 and put them on a leash. Since i'm on a rebreather, I'd just as soon leash the deco bottles as well or nose clip them in, but again as a cave diver, we don't carry those for very long.
In sidemount it doesn't really matter since we tend to split bottles between the left and right anyway, so they all end up on the right.
In open water for big dives, I still use a rack and all bottles go on the left per the "standard".