It's almost always preferable to use a ferrule. If for no other reason than confining erosion damage caused by the sharp end-on wires to a replaceable part. However some levers don't have the room for a ferrule in the fittings. It's an easy decision to make. If there's enough room - the fitting has a bigger ID than the housing OD - use a ferrule. If the housing is a slip fit to the fitting than don't (because you can't anyway).
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