It all depends on how and where you drive. If you have nice roads and never go wheeling, the factory ones probably work just fine.
I bought an 18 with the factory Rancho's. In my opinion, they were dangerous for where I drive. I wouldn't let my wife drive it until I changed the shocks. I almost got bounced off the road more than once. There is no rebound dampening and when the springs load up there is nothing to slow them down when they let go. I swear the front end would jump off the ground sometimes when 4 wheeling. And I take it pretty easy most of the time and to try and let the suspension to it's job. At 4k miles I put on 5100's, I'm not a bilstein fanboy but seemed like the best thing going for the price. No more jumping up in the air, I can drive over a dip without being bounced off the road. No it doesn't ride like a car anymore, but its a truck. I have a car when I need a car.
These factory Rancho's are not the same as real Rancho's I've had before. I had Rancho 5000's on a 81 Cherokee and I put more than 200K miles on the shocks. Hardly any on the highway. They did the job, no complaints. When I saw the Rancho's on the new truck I thought, great, I don't have to change the shocks for a while. Not the case. These factory ones with the painted boot covers are something else. Maybe the new 5000's are junk too, I bought mine in the 80's.
I also swapped the 20in rims for 17's when I did the shocks. After riding around a bit I realized the 20's weren't going to take much off-roading and still look good. It rides much better now, even with the Mickey Thompson ATZ's.