I read an article that contained the results of an American army medical study into back problems in Bells, a few years ago and it debunked the idea that vibration was a factor. Basically, they rigged a platform on hydraulic jacks, fitted a seat and controls and then had a TV or Video game (can't remember now) for the test subjects to occupy themselves with. One group did the test with the jacks in action to simulate a Huey's vibration and the other group sat with no vibration. The results were the same and it was considered that posture (bent forward and down to the left slightly) and imobility were the dominant factors.
Having said that, I'd rather have a smooth ride than not.
Another thing that greatly affects comfort is the state of the seat cushions; half the time you are sitting on about half an inch of foam rather than about 2 inches, as a result of thousands of hours of compression from backsides great and small. A new cushion can transform the comfort levels.