After quite a bit of experimentation, I use an LED light table + glassless negative carrier + copy-stand + DSLR + macro lens to digitise my 6x6 and 6x7 negs. In my experience the negative carrier works much better (and faster) than using glass. True, the glass holds the film perfectly flat, but normal glass gives terrible Newton rings. ANR glass doesn't always prevent all Newton rings either (I know, I tried it with very high quality ANR glass plates), and furthermore causes a very noticeable softening of the captured image. Any tiny amount of non-planarity in the film when in the negative carrier is easily compensated by stopping down the macro lens to f8-f11.
It is also much easier to minimise the amount of dust with the above setup, as you only have two surfaces to deal with (front and back of film) rather than 5 or more.