Nodal point, entrance pupil, whatever. For my camera, the tripod screw is directly below the lens, and my mount holds it sideways (in portrait mode) in a position that's close enough. Tilt and pan parallax are small enough to ignore except for extremely close-up features.
I once read a no-math, empirical way to find the "optical center of rotation" for your camera or rig. You take your rig outside and plant it a couple of feet from a chain-link fence, with something obvious (like a building) far away on the other side of the fence. Looking through the SLR viewfinder or the LCD display, identify a spot on one side of the image where a crossing in the fence is right next to a feature on the building. Now pan the camera so the crossing-and-feature combo is on the other side of the image. If the relative positions of the wire crossing and the building feature do not change, you have found your optical center. If they do change, adjust your rig and try again: if it gets worse, adjust in the other direction. Repeat until satisfied. Perfection won't matter unless you do a lot of interior panos with close-up objects. The same trick works for tilt as you move your crossing-and-feature from the bottom of the image field to the top (or vice versa).
