Setting 0 in pitch black.
Setting 1 in energy conservation mode
Setting 4 when I don't care about energy
Setting 10 when I want to see backlight in broad day light.
I have to admit that the backlight looks tempting, and you want to put as much of it as you could possibly bare (kinda like sugar).
But to stay healthy, I find any setting beyond 4 is not really necessary, as when the environment is light enough to blot out the screen light at setting 4, then it's light enough to be reading without backlight.