I feel that it depends on whether you are aware of your own vocal functions enough to make a physical decision that is good for the voice - I can think of ways of whispering that would be harmful, and, after a while, probably even start to hurt. But then there are other ways to whisper that are much simpler and less traumatic. I prefer speaking extremely softly - when whispering, the strain can probably come from the need to articulate sounds without vibration, so we push more and can cause more damage without realizing what we're doing.
Very interesting. Copied here from Ken Schatz's Facebook.