FS and SF corners are often not "realistic" (not even as
realistic as the sandbagged SS corner). You cannot, for
example, get a max-spec Tox on the N and a min-spec
Tox on the P channel devices. Same for poly etch (deltaL)
and active area etch (deltaW) and so on.
So putting yourself on the hook for timing closure and such,
for unrealistic events, would add only pain.