I would think it easier to add the pseudo grain when making the print, as well you would have an unaltered neg to use in other ways too.
WJ recommends a polyester filter between the rails for opaque filters to uncomplicate the shooting and speed up the process. You can see to focus and shoot immediately and you can use any lens you have without having to buy new filters or adapt one size to another. Makes a big difference to him as he takes his camera along on backpack ski trips where fiddling with a filter while shooting can be a very big chore. Its really valuable for wide angle lenses that tend to vignette with any filter in the way.
That said I don't have that because I haven't figured out how to do it with a Bronica SQAi. I will sooner or later but now I get satisfactory results with a 29 red filter.