I'm going to put one in, as I think it gets them used to the roost. I got some teenagers from my sister and they had never been exposed to a roost. They kept trying to sleep in my nest boxes and had thoroughly soiled my nests! I had to pull up my nest box roosts each night just to keep them out. They finally learned to roost after that, but I'm going to start my babies out now and adjust the roost up as they grow.
Different size dowel rods should be appropriate and can be changed out as they go. Most folks on here advocate a flat roost, but my chickens like a round roost for better grasping.