My experience with Rodinal is indeed that exposure (or density) will strongly affect granularity. Results can be rather unobtrusive even with a traditional 400 speed film like APX400 and 1:25 dilution, as long as you expose it sparingly, which of course requires that you chose subjects and lighting situations that tolerate that kind of exposure in the first place. Grain will be present, but not as annoyingly so. I've did some comparisons recently with the same subject, lighting and film, and to my surprise the difference between Rodinal 1:25 and Xtol R in grain size is smaller than one might expect. But if you overexpose it, overdevelop it, use it for high contrast situations and/or subjects that include large uniform areas of higher density, the same film will appear as a grainy mess. It is really the question of choosing the right tool for your job. Also, I agree for hybrid users the scanning process and especially the light source of the scanner will have a big impact on how the grain appears in the scan.