Is ? a distinguished character in UUCP mail?

A friend of mine was worried that the use of the =????= encoding of headers
would break UUCP mailers.
Is there any reason for this worry?
(His solution: Use / - I know that this breaks some UUCP mailers if used
in the ADDRESS.........but not in Freeform text, I think....)

                   Harald Tveit Alvestrand