Lower temperatures have no effect on a film's grain size with any developer, nor do higher temperatures of up to about 28-30°C it's a total myth and been proved many times by research at the major film companies.
Originally Posted by piu58
However because of the free Hydroxide in Rodinal poor temperature control across the process cycle can result in micro reticulation, the hydroxide softens the emulsion, it can be worse at higher temperatures as the swell of gelatin is also temperature related working a lower temperatures can cover up bad technique. The results of micro reticulation which is a surface effect is more apparent graininess in prints and scans, but the actual film grain is normal.
Agfa used to recommend mixing up Rodinal with a weak sulphite solution if you needed to prolong the life of a working solution this also helps give finer grain.