"Kansas City" alone is often ambiguous, and so I would not include it on a list of "dateline cities." For every person who thinks it's obvious that a reference with no state means the much larger one, in Missouri, there's probably, oh, I don't know, a third of a person who thinks it's obvious that it means the one in the eponymous state.
But here's an example of why the dateline-city thing, like all style guidelines, is only a guideline:
RAYTOWN, Mo. — Police say that the bodies of four people have been found in an apartment near Kansas City and that they suspect foul play. Police are trying to confirm the identities of the dead, including two preteen boys.
No need to say "Kansas City, Mo." when (a) we already know the suburb is in Missouri and (b) if it's near the one in Missouri, it's also near the one in Kansas.