For years, doctors have been saying that to prevent heart disease, patients should pay attention to both the so-called bad cholesterol, or L.D.L., and the good cholesterol, or H.D.L. The good, they said, can counteract the bad. But now, some scientists say, new and continuing studies have called into question whether high levels of the good cholesterol are always good and, when they are beneficial, how much. While some heart experts are not ready to change their treatment advice, others have concluded that H.D.L. should play at most a minor role in deciding whether to prescribe cholesterol-lowering drugs. In the...