Why you should use F8 aperture for taking 360° panoramas

The attendees of my workshops ask me quite often why I use the F8 stop for taking my panoramas. I’ve got several reasons to back up this decision:

• It is necessary to choose an aperture F-stop and to stick with it during shooting, preferably for the whole shooting session (not just one panorama). 
• The lens I use (Sigma 8mm F3.5) has the sharpest image at F8. I don’t mean the depth of focus, which would be the highest with the highest possible aperture. When I focus between the F8 and F16 setting, this lens gives the best picture from the whole spectrum of choices.
• F8 is high enough for me to be able to focus from several decimeters (1-2 feet) to infinity.
• F8 is low enough to ensure I get enough light on my sensor even with more difficult lighting conditions. In extreme circumstances (evening, night) I set the aperture to even lower value to shorten the exposure time if needed. The depth of focus is therefore sacrificed in favor of shorter exposure time, especially when shooting handheld.

What is the F-stop that you use for your panoramas in normal lighting conditions. And why?