The couple of airlines I have worked for have always had an SOP of flying the approach at the lowest approach capability that you have. So when Low Vis Ops are in force I would always fly a CATIII B No DH if the airport is equiped. The actual RVR is only important in working out what 'reversions' you have i.e. if you have a downgrade from CATIII to CATII, do you have the ability to carry out a CATII with the current RVR?