With any 90mm lens on 6x17, you're probably going to want to correct the falloff to some extent, either on the lens or in the darkroom, particularly if you shoot color transparency film. If you shoot B&W and don't mind lots of falloff, maybe it won't matter as much, but if you correct it to some extent, it gives you more compositional flexibility. More falloff tends to drive the subject to the center of the frame, or stated alternately, when the subject appears in the falloff zone, you may just find that image to be unsuccessful when you look at the contact sheets or transparencies, because these things can be hard to judge on the groundglass.

Even with a center filter--even the filter supposedly "matched" to the lens--the correction isn't 100%, but might be visually acceptable. If center filters actually corrected falloff fully, which they could only do at a specified focus distance and aperture, they would usually be around three stops and would presume a shooting aperture of around f:22. At one point, such filters were made, and they didn't sell particularly well, because they were impractical, so most of the available center filters tend to correct about 1.25-2 stops at f:22 and leave an aesthetically acceptable amount of falloff. Heliopan may still have a 3-stop center filter. I have several wide lenses, and I haven't had any particular problems like banding with "unmatched" center filters, and I haven't found it necessary to purchase "matched" center filters with only slight differences in density.