My lens of choice with 6x17 is a 75mm Super Angulon, it's a great lens to work with. I find this is a natural standard lens for the format, remember that in the vertical plane it's close to being the common standard lens for 6x6, yet it has good wide field coverage for the panoramic format.

In comparison I found that I never used my 90mm as it was too restrictive, I don't shoot in wide open landscapes, my 65mm Sopere Angulon won't cover 6x17 and is tight on 5x4. Of course the newer SA 72mm XL would be ideal. Like Lachlan I find with negative film I can get away without using a center filter, which would actually be restrictive in practice anyway as I often have to shoot hand held, and even in bright light can't afforda loss of effective speed.