There is of course many things that can cause stitching errors even if your nodal point is perfect. But since you asked the question of how to get "perfect" results we must look at what it takes to get a "perfect" stitch. And since we are talking about "perfect" we must look into the situation with a little more detail.
In fact, it is not possible to rotate "exactly" on the axis of the "true nodal point". We are actually rotating around the perimeter of the nodal point. The nodal point is the point at which light converges inside the lens. This IS a point. But the "entrance pupil" is round and is at a vertical orientation. Take a coin and rotate it on its edge. You will notice the outside edges of the coin rotate in an arc with this arc decreasing towards the center. So you are actually rotating around both the entrance pupil and the nodal point together. Although the distance from the entrance pupil and the nodal point is very small it is enough to cause light waves to be slightly shifted as you rotate the camera.
With this inherent aberration in mind realize that stitching errors will occur but they are normally hidden due to the blending and accuracy of the software used.
What happens is any other factor that can cause a stitching error will magnify this inherent problem.
The most important of these possibilities is the level of the camera. It must be perfectly level horizontally as well as vertically.
Horizontal level is easy to obtain by using a steady tripod and added tripod leveler. It should be noted that you should never re level the camera at any point. Once you have established level on the first position of the frame set that it where it should stay.
The camera itself is mounted vertically and one should use a hot shoe bubble to level the camera. Both the spirit level on the Agnos bracket and the hot shoe should show level before you begin to shoot.
The quick release mount that attaches to the camera is secured with a screw but is held in place by a cork friction plate. It is possible to cause slight vertical rotation if you are not very careful when pressing the shutter release. When rotating for the top and bottom it is easy to grab the camera and when pulling up or down and slightly move the quick release plate.
These points are related to "mechanical" issues that are operator error. Now, I am splitting hairs here but (we are talking about perfect) it does happen especially if you are shooting a lot of scenes at once. You should check every thing after a few scenes to make sure all connections are secure. If the tripod leveler unscrews a little then you will have a slight unbalance with the rotator. So check everything.
The "scene" you are shooting will also effect stitching. You may notice that in scenes with lots of detail and contrast the stitching will come out perfect. On other scenes with less detail or with large areas of solid color or even large areas with texture like sand, grass or trees the image will have more stitching errors.
A water scene on the shore where there is all sky and a constant level of texture like sand or grass all around you will cause stitching errors more than and interior shot with lots of detail. The software relies on sharpness to calculate matching points. When there is less detail (contrast between pixels) it is difficult to accurately calculate these points automatically.
Although your lens has a huge depth of field the DOF can be moved slightly depending on the aperture selected. IF you are not using infinity focus you will not be shooting at the hyperfocal distance. If you focus on something farther away the image will still be in focus but you will be moving the DOF ever so slightly and this translates into the image being rendered onto the CCD in different levels of sharpness.
NOTE: To be even more specific. This phenomenon is related to the circle of confusion. See Daves post on what is COF and DOF anyway?
This is very, very small and would not be a factor in still photography. But IF you have one or two frames that this happens on, the sharpness between pixels makes detail areas located by the software bigger or smaller due the slight unsharpness of the pixels picked. This changes the locations of the matching points between frames. The software thinks they are in the exact position but they are in fact not. This will cause a stitching error but should only be noticeable on the edge of something sharp like a baseboard. On solid surfaces or areas with a matrix of detail you won't notice it.
We also can't rule out temperature changes either. If you are inside for a long time at 72 degree temperature and move outside to 95 degree temperature your tripod, mounts and the bracket are going to expand some. This expansion and contraction will technically effect the nodal point position. It will take a few minutes for the equipment to stabilize. If you start shooting immediately you will have different nodal points on different frames or event different nodal points on different scenes.
For the software to stitch a mathematically perfect stitch each frame must be rendered in exactly the same position on the CCD. Anything that changes this will result in some error at least at the math level.
Tolerance is what we are talking about. What is the error factor of the software? There is one of course but this error is within the acceptable range of "perfect". Anything that increase the inherent error factor to bring it out of the acceptable range will result is some amount of stitching error. Whether you can find it or not is another question. As the errors get larger it gets to the point where you can see it.
I really think that most stitching errors result from leveling problems and details in the image. The sharper the image is the better the stitch will be. Aperture plays a big role in this. With the sigma at 8mm you should be shooting in the F7 F8 range and controlling exposure with shutter speed.