I've never trusted them, especially not after an incident departing Vienna when a KLM agent was trying to nail me for an overweight carryon that I knew wasn't overweight. She finally backed off when I pointed out that her scale reset to 2 kg. (approx 4 lb.) when nothing was on it. Around Los Angeles it turns out that most scales are either accurate or err in favor of the flyer at three major airports: LA Times: How accurate are airport luggage scales?