Matt Grum mentions in a comment on an answer related to locking parameters when shooting a panorama that it is beneficial to zoom out on the middle section of the image while shooting a single row panorama when a rectilinear projection is applied in post.
This would mean that one focal length is used for the side of the image, a slightly shorter focal length at the middle section, and again the longer focal length at the other side of the image.
The goal is to decrease the bow-tie shape that a wide, rectilinear panorama has and hence do less cropping on the final image.
How much should one decrease the focal length between the side parts of the panorama and the centre part such that the bow-tie shape is minimised and less parts have to be cropped off?
Are there any other benefits to zooming while shooting a panorama image?