Out of curiosity, are you using a center filter or dodging and burning to even out the illumination? When I was shooting 6x17, I found I always needed a center filter with my 75mm/4.5 Grandagon-N, even for B&W.
David, I don't own a centre filter but one would be useful on the rare occasions when I'm able to use a tripod, but I can't afford the drop in exposure of a filter when working hand-held. So yes I'm dodging and burning, at the same time I'm shooting digital for publication & I have noticed how markedly the lighting changes on stitched panoramic shots across the image.