I have seen this brought a few times in our company publications. Supposedly, NW tracks who sends complaints and deals accordingly. For example, if you constantly complain about little things and then have a major problems they will still take your problem seriously. If you are just a constant complainer, they will continue to send you a "thanks, hope you fly us again" letter.
There was time (and I suppose it still exists today) when pax were complaining in an effort to get miles or something for free even when the employee was not at fault.
I think there was thread on this forum about this. Someone that posts here was labeled a constant complainer by NW and they posted the correspondence.