For LCD's/LED's, a simple answer:
Especially in dimmer to dark rooms, always turn your backlight down as low as you can tolerate.
Lower backlight levels accomplish a number of things.
All else being equal, lowering the backlight setting typically lowers energy consumption. This is not a bad thing, right?
It also can extend the useful life of your backlight source. The harder you drive your backlights, the faster they will wear out.
In a dim to dark room, the lower you have your backlight, the less chance you have of inducing eye strain.
Lowering your backlight typically gives you lower (i.e. deeper) black levels, admittedly, at the expense of some light output. This is a good thing, within reason. You don't want your backlight so low that you lose all your contrast and details are obscured.
Setting your backlight lower often helpls to minimize flashlighting, clouding, blooming, backlight bleed, and other screen uniformity issues.
Setting your backlight lower will also keep your display running cooler, which also helps when it comes to screen uniformity issues. For example, sometimes clouding becomes more noticeable on LCD's after they heat up and get good and warm.
In a dark room it is simple. Don't start at the top and work your way down, but start at the bottom and work your way up. Start at the minimum backlight setting. It may look too dim and some shadow details could be obscured. In this case, slowly raise it up until the light output is sufficiently pleasing to your eye for extended periods of viewing.
In a bright room, you can get away with using a much higher backlight setting, because flashlighting and clouding and such are not as noticeable in a bright room, and as long as the ambient light reflecting off the panel is greater than the MLL of the display, your perception of how deep the blacks are will be more a function of how dark the light absorption filter is. So you can still maintain the perception of deep blacks, even though the higher backlight is elevating your black levels (making them glow grayer).
In a dark room I found 3 or 4 out of 10 was plenty of light output on my Sammy B8500. 6 or 7 may be more than enough for the 2010 models that have a backlight that goes to 20. Typically, as a rule of thumb, you should be at less than half the max setting in a dark room.