You should rate the film at whatever speed gives you the amount of shadow detail you want. There is no right and wrong.
Try shooting a roll at 25, 32, 40, 50, 64, 80, and 100. Develop the film, print the negatives, and see which gives you the amount of shadow detail you want. After you pick that, shoot one roll at your favored speed, and now dial in the rest of the tone spectrum by adjusting developing time. Once again, print the negs to see where they fall into place without much effort.
Hope that helps.