It may be graphics related as well. Since the 1.4.2 patch, I haven't found the problem any more. But I just tried it on my server PC, which has a lower quality card, and it does stretch a little. But nowhere near as much as it used to.
As I said above, they stretch in real life if you apply too much pull. If you want realism, keep the power right down. No tug would aply that much force to a hawser. You'd kill someone when the haswer snapped or the deckplates came away from the ship!