Hi HM - your question depends on what you want to "do" with your files. There is little doubt in my mind that the RAW processing on your earlier photos will be "better" with DxO. In fact you always have that option (going back and starting over, I mean) with any new application that comes out ... if you have kept the orginal NEFs. So if you have some great captures that you feel would be enhanced and do not need a lot of spot/local adjustments I would invest the time in re-processing them ... but perhaps not all of them.. I reserve my use of DxO Optics Pro for my "best" images which are going to be displayed or printed etc.
When words become unclear, I shall focus with photographs. When images become inadequate, I shall be content with silence. - Ansel Adams