I usually flip it all the way off. If I have it on it’s maxed.

Generally in the middle, higher if it’s a 2D game using subtle 3D for the UI.
I’ve also had cases where I had in barely on just so i could get the extra resolution. Some homebrew looks nicer like this.

All or nothing. I generally keep it on, though.

New N3DS - somewhere between half and max setting for 3D games and barely turned on for 2D ones. 3D really needs depth to pop while 2D starts to look too much like a paper cutout with setting too high.