I shoot a 65mm f8 Ilex Acugon on 4x5 and don't use a center filter for it. I only shoot B&W but haven't seen a need for the filter. The f5.6 will have even more coverage and using it to shoot 6x9 instead of 4x5, I would think that a center filter would not be needed unless, maybe, you're using a lot of movements.
I have an old Sinar 65mm f8 Super Angulon (barrel-mounted, with Norma Rabbit Ears), and for 4x5, I prefer to use the matching Schneider Center Filter. Shooting 4x5 chromes, it -does- help. You can clearly see it. Despite the fact that it is a teeny little lens, I really like it alot.
I use the S-A 65mm f5.6 on 4x5, and have two of the the S-A 65mm f8s that I use on 4x5 and 6x9. On 4x5 I almost always use the centre filter, and it is a necessity for almost all work on colour transparency. On 6x9 (with the f8) I hardly ever use it - for B&W you probably don't need it at all, for colour slide it might be useful on ocassion but certainly isn't going to be an essential.
Both 65s are very nice lenses. The f5.6 will give you masses of movement on 6x9. I bought mine from an architectural photographer who was retiring and had used it on 6x9 for a good proportion of her work on interiors.
I usually use the center filter (now that I have one) with my 65/8 Super-Angulon on 4x5", but I haven't used it on 6x9 enough to know whether it's necessary.