I added caster to mine and the jury is still out for me whether it helped or not. I added a steering stabilizer at about the same time, and while it definitely tracks better it's hard for me to ascertain which one is responsible. The combination of the two makes the steering a little heavy, and I'd be inclined to try one or the other for awhile if I had it to do again. That said, the increased caster didn't hurt the ride and my rig handles pretty well. Front and rear sway bars were also a big (and expensive) improvement.
2004 National Dolphin LX 6320, W-22, 8.1, Allison 1000, Front/rear "Trac" Bars & Anti-sway Bars, Sumo Springs, Roadmaster Reflex Steering Stabilizer, 2005 PT Cruiser Toad