Scuba diving upside-down is a newfound thrill, ever since participating in a coral reef restoration project. We had school-age children snorkeling above us, watching what we were doing. A few times, I rolled onto my back and snapped a few photos of them on the surface.
Since then, I have engaged in all kinds of underwater acrobatics. I roll onto my side to photograph my dive buddies. Also, I roll onto my back to photograph divers and other interesting things above me. For example, when drifting along a wall, divers usually end up at different depths and schools of fish swim directly above.
I don’t think this affects my buoyancy or air consumption much. I do the same thing I normally do, except 180° rotated. I sometimes roll over and then tilt upward to photograph divers behind me, and that is definitely bad trim, but that is very brief and too much fun to stop.
For the record, I stay on my back and sides for very short periods. I want to make sure I don’t collide with anything or anyone. I only take a photo or two or three, and then resume proper trim. Or, if I am going to observe something for a while, such as a school of fish, I’m more likely to go vertical for a while, which is what other divers seem to do.
Anyway, I’m far less concerned about the length of my dives, in regards to air consumption, and far more interested in having awesome, fun, yet-still-safe dives.