The format follows consistent rules, but the application differs in different U.S. states.
In general, the names of cities and towns (or incorporated municipalities with other names, such as boroughs Pennsylvania) are unique within the state (this is something that state governments control), so the category name will be simply [[Category: City, State]]. Important exceptions exist for a number of states where the same township name can exist in multiple counties -- for example, a number of U.S. states have multiple counties that include a Washington Township. In those states, all townships (or equivalent entities if the state uses a different term) need to have the county name in their category name ([[Category: Washington Township, XYZ County, State]]). I think there are a few instances around the country where the county name might be needed in a category to avoid ambiguity in a situation where two non-township communities in the state have had the same name -- for example, at different times in history, but it wouldn't make sense to create a general rule to accommodate a few oddball situations.
And since a county category includes (as subcategories) all local places that are fully or partially in that county, a person should not need the county name to be part of the category name in order to investigate the contents of categories for other parts of the county.