Canadian Power and Sail Squadron teaches 7:1, but as others have said, swinging room is the realistic yardstick. In places where you can expect to lie in the same direction all night (i.e. in the trades), all the scope you can manage is best. If, however, as here on the West Coast the bays are small, generally deep and winds & currents go back and forth all the time, swinging room (and the practice of those already there) severely limits your options. Around here running sternlines ashore is a common practice to limit swinging but that too can be problematic in a strong cross-wind or cross-current situations.
Use common sense and keep a good eye (and ear) out for trouble overnight!