This is just one person's considered personal opinion - take it for what an opinion is worth. Personally, my mantra when exhibiting work is "consistency, consistency, consistency". If you can avoid variation in film grain, I would avoid it. If you cannot, then work with it so that it becomes part of the consistency. Don't have just one or two images out of the 20 or so have a visibly different grain structure. If a third or more of the images are of a different grain, then fine. A third is a very rough figure, not to be taken as any kind of specific rule. In the end, I think you'll know the harmony when you see it.