There is a reason why the manual says ‘should’ about use of all runways. Should means it is highly recommended and the reason it isn’t ‘must’ is only because of places where it just isn’t possible to use both. Other than exceptions where it’s physically not possible to use them all, controllers are encouraged to follow the recommendation in the ATC manual.
Must, “The use of must or must not are considered MANDATORY and strict compliance is REQUIRED”?. Should, “The use of should or should not are considered ADVISORY and compliance is STRONGLY RECOMMENDED”. You have to pay attention to the differences between must and should when reading the manual.
References: ATC Manual