Not anymore so than laying down. There's no such thing as spinal decompression or being able to stretch space between vertebra....or any reason that you would want to do so. Disc degeneration is normal and permanent and mean old Mr Gravity is going to eliminate any space you think you created by hanging, laying, or wearing silly chin straps the minute you stand up...or sit in a chair.
True shoulder impingement is a very rare condition but the phrase is used colloquially to describe any pain or tightness associated with full range of motion in the shoulder. Its mostly caused by tendonitis or small tears in the muscle that make up the rotator cuff. Dead hangs don't do anything but allow a person to force a full range of motion into a shoulder that is perceived not to have full range of motion. There's better and longer lasting ways to reduce and eliminate this pain, but not as simple or easy and just settling for short term relief.....but relief is a good thing and if hanging, or any other type of placebo provides short term relief there's no reason not to do it as long as your not causing any damage in the process.
If you're going to do dead hangs make sure you shrug your shoulders up while hanging. This causes the scapula to rotate out of the way and minimizes the chance of doing real damage by pinching the soft tissue between the bones in the area of the shoulder joint (the head of the humerus, the distal end of the collarbone, and the distal end of the scapula). When you shrug your shoulders it rotates the scapulas up and in, towards the middle, pulling the knobby parts of the AC away from the head of the humerus, so that there is more space between them and less chance of "impinging" soft tissue.
