You would have a tough time living/working in Beijing and consistently adhering to that policy these days; during rush hour in the CBD, it can often take over 30 minutes to find a legit taxi, and when you do, you'd better be prepared to move quickly and sharpen your elbows (think of the opening scene in Planes, Trains, and Automobiles). Naturally, I always try to get a real taxi, but don't think twice about grabbing a 黑车 if one passes by during a taxi drought, though I always reject the guys that quote insane prices (and are unwilling to negotiate); I'll gladly pay a y20 premium, but not much more than that.