yes these are important muscles and even deeper is the rotator group together they are the first layer of muscles.
i believe that is is less important to train them (if possible at all?) than to make sure they do not tense up because of an underlying subluxation (small misalignment)
as with all muscles they hold and stabilize the joints and if the joint (vertebra) gets out of place a little they may assume this still as correct position and ´fix´it there which is followed by an un-natural tensioning that becomes chronic after 2 month and the subluxation will not go back by itself. This will in turn lead to a ´pinching´of the nerves with pain sooner or later and a negative influence also on a nerve called ´ramii communicans´ that may cause on the long run trouble on the organ level as well.
a re-alignment should therefore be the first step followed by muscle work.