Just to see how much difference it makes I’ve tried round tree branches, 2x4’s on edge, and 2x4’s flat. I’ve concluded that people care about this a lot more than the chickens do. Do whichever is most convenient to you. The chickens won’t care.
If you use the 2x4’s I suggest sanding the corners a bit. Some people will say this is to make it easier for the chickens’ feet to grip, but I do it to remove splinters.