There are at least three ways that a tightened mounting bolt can allow for free rotation of the derailer about the pivot bolt.


1) The bolt features a stepped-up diameter above the threaded end, which bottoms on the hanger, thus preventing tightening torque from binding the "B-knuckle" at the mounting bolt.


2) The bolt features extended threading for a locknut on the reverse side of the hanger mounting hole. The Locknut locks the bolt without the bolt pinching the B-knuckle, so the derailer is free to rotate about the mounting bolt.


3) The mounting bolt is threaded internally where it is threaded externally, allowing a set screw to be tightened inside of the threaded end of the mounting bolt, thus expanding the bolt so it cannot move within the hanger mounting hole. The derailer again is free to rotate about the mounting bolt which effectively acts like a rigid part of the frame without having it be tightened down against the derailer's B-knuckle.


I use a Duopar derailer on my Trek 510 with the mounting bolt tightened down on the derailer's upper (B) knuckle.
The washer tab broke off long ago as the washer is hardened, and thus brittle. You cannot bend these things!
I have great difficulty removing the rear wheel. I first remove the QR completely and then "finesse" the wheel out of the frame without loosening the mounting bolt. I'm sure this would be easier with vertical dropouts, or with even longer chainstays than this sport-touring bike already has.