Here's my experiment using the AFS 18-200mm on the D5100.
All the photos, with or without the tubes had the lens fully extended to the 200mm position.
First, the flower taken at the closest focusing distance with no tube attached.
Now the closest focusing distance using the middle 20mm tube.
The 20mm tube can stay on the lens if you want a further view. Here I was 15" from the flower. The lens was still at 200mm.
Here's the flower using the longest 36mm tube. The front of the lens was nearly touching the flower. With this lens I could not stack tubes without touching the flower.
This was a hastily set up shoot. I did not use a tripod and I had to manually focus the lens. I kept VR on. The lens was set to f8 for all photos. This flower requires a large dof to keep everything in focus. I would normally use a tripod and the lens stopped down to f11 and maybe f16. In this case the flower was not in a position that a tripod would fit.
So here I used the 20mm tube and set the lens to f16. I had to bring up the ISO to 3200. I also increased the contrast and added some sharpness. Handheld, manual focus, VR on.