coding standard for allowed arguments of predicates

[Roland Winkler]

Is there a coding standard for allowed arguments of predicates?
I have always assumed that predicates should accept any type of
argument;  but I cannot find this documented anywhere.

The context of my question is `iso8601-valid-p'.  It assumes that
its argument must be a string.  Is this a bug?  In my usage example,
the argument of `iso8601-valid-p' could also be nil.

If, indeed, predicates should always accept any type of argument,
I suggest to document this in the elisp manual.