Bryan, thanks for the reply. I appreciate it.
I think that when stitching across the full width of the image circle, with the lens fixed and the back moving, the fall off will probably be significant, hence the need for the filter. While it could be fixed in PS, the time factors and tolerances involved may well indicate that using the filter in the first place would probably be a better solution, but like you, I'm thinking theoretically, not from experience.
Thanks again for helping confirm my views
Jon