I agree with others who've said that if one can get by without the tubing, that is the best option; as vipermann7 said it's just something else to worry about and that might fail at an inopportune time. But, I also agree with howhill1 about the noise from the tubing. How can one know that "rubber tubing is making enough noise to help the deer 'jump the string'?" How much difference in sound is added by the tubing compared to the sound made by the bow itself and any add-ons (e.g., sight, quiver, etc.)?
Also, it was stated in the original post that though one of the deer ducked, it was hit in the liver. I don't see how ducking would lead to a liver hit. It might if the deer moved forward since the liver is several inches back from an ideal shot placement. Was it walking at the shot?
I also agree with howhill1 that maybe the deer was on high alert as a result of the detection of scent, other sounds, etc. Unless it was alerted, sensing it was in a dangerous situation, and already about to bolt, it seems unlikely that at 17-18 yards that it was able to react and jump the string enough to make a good shot miss by more than a couple of inches. My math shows, depending on speed of the arrow, at 18 yards the arrow would get there in no more than 0.20 sec, while the sound would reach that distance in about 0.05 sec. In a relaxed deer, a significant amount of the difference would be consumed by evaluating the threat value of the sound before mobilizing the appropriate response. However, in an alert deer, it may begin to respond when the sound arrived.