HI Stephen,
For what it's worth, I have used an 80mm Rodagon for 6x7 with great results at f/8 - especially if I want a touch more magnification. Typically, however, I use the 105mm for 6x7 because I am usually making smaller prints (ie. 8x10) and printing close to the full frame of the negative. I have found no practical difference in image quality or light fall off in my work between the 80mm or 105mm for 6x7 work. Others mileage may vary. I might add the 6x7 negatives I have been enlarging are shot on a Pentax 6x7 with 90mm and 55mm (new version) lenses - so they are very sharp.
Hope this helps.
Best,
Paul