I installed my DSS at 27K or so. No significant improvements, but from what I can tell, I believe it depends on the type of mlles driven and how many. Higher mileage trucks seem to experience the most improvements.
I hope over the long-haul, it will keep things tighter.
But as mediccummin said, there are many things that contribute. I had the trackbar replaced BEFORE the DSS and that made the most improvement. Then about 7000 miles later, I'm wandering all over the road again. The dealer said trackbar was OK, but found out the intermediate steering shaft was waay out of spec. They put a new one in (under warranty) and it drove really nice again. All this was done prior to 36,000 miles. Kinda dissappointing all these things wore out so fast. And most of my driving is empty, in-town.