Deco bottle set up with Stealth

[Aotus]

is there a good reason for stage or deco below main cylinder or is it just preference
how do you maintain sideways trim with deco bottle on one side, what about putting it on your back like a standard set up

A lot of (most of?) sidemount divers are also cave divers. To minimize the chance of accidental contact with the ceiling in caves with delicate speleothems (and to fit through restrictions), most prefer keeping the stages below their primary tanks. In FL, where a lot of caves are high flow, at least part of the year, and scooters are common, many have found that top mounting stages helps them streamline. Some also choose to top mount their stages if they are particularly concerned about disturbing silt below them. By top mont, I mean stacked on top of the primary cylinders, not on the diver's back. Putting a stage on your back would defeat the purpose of sidemount diving in the first place.

Edit: Re balancing. swimming for long with an extra cylinder on one side is annoying, but it can be managed with a drop weight on the other side or even just positioning you arms to counter the weight of a stage on one side. Typically, you aren't swimming for a real long time with only one full stage, and on your way out it is lighter.