Hello Rob... I hope I'm not too late with my reply. I haven't been able to get into the forum for a couple of weeks... maybe a China thing.
While the 90XL does cover the format and we do support it, when asked it is not a lens we normally reccomend. Of course light fall of is problematic (though cureable), but more importantly the 90mm focal length is EXTREMELY wide on 624. Finding subject matter that holds up well is not all that easy... those georgeous mountains in the background can become quite diminutive and lose their drama quite easily. BTW - my comments apply equally to the 617/72XL combination. From my own experience the best WIDE lens for the 624 format is the 110XL. This is echoed by others as well. Take a look at Micheal Hubry's work, which appears in our Users Gallery (http://www.fotomancamera.com/gallery.asp)... he uses the 110XL exclusively, producing some truly beautiful images. For B&W with the 624/110XL combo you could likely get away without a CF, but for color transparency materials I would reccomend using one (fortunately it has a 67mm filter thread). This is likely the case for all lenses used on 624 of 150mm or less (excluding perhaps the Schneider 150XL). I've used the 90XL, 110XL, 120 Super Angulon, Nikon 120 SW and Fuji 125 SW... in all cases using a center filter was required to even out exposure in sky areas, something there is usually a LOT of on 6x24. Once again... this is for color transparency films. I have yet to work with B&W on 624, so I can not give you first hand advise in this regard.