Question regarding fwrite() and fflush()

In a recent reply here, I posted some code that used fwrite() to write
binary data to a file. One version of this code used mmap() to access
this binary data in-situ, and I wonder if I should have called fflush()
before calling mmap().

I understand the where to use fflush() on standard text formatted output
(where you use fprintf(), etc to externalize the output), but I don't see
anything my copies of either the ISO C or POSIX standards that say that
I /should/ use fflush(), but neither do I see anything that says I /need
not/ use fflush().

So, wiser and more experienced minds, should I use fflush() in this case
or not?

--
Lew Pitcher
"In Skills We Trust"