Are prototypes required for all functions in C89, C90 or C99?

Question

To be truly standards-compliant, must all functions in C (except for main) have a prototype, even if they are only used after their definition in the same translation unit?<

Answer 1

To the best of my knowledge (in ANSI C89/ISO C90), no. I am unsure about C99; however, I would expect the same.

Personal Note: I only write function prototypes when...

1. I need to (when A() calls B() and B() calls A()), or
2. I am exporting the function; otherwise, it feels superfluous.