You will find that different people have different opinions on this. Some are absolutely adamant that you have to have a flat 2x4 or you are just cruel. Others are equally convinced that your roost has to be round (like a tree branch) or you are torturing your birds. A lot of people use 2x4's on edge or 2x2's but don't seem as passionate about this.
I've tried 2x4's flat, 2x4's on edge, and tree branches, mixing them up and moving them around to try to negate the issue of position. From what I've seen the chickens don't care about the shape of the roost, where it is in the coop is what is important. Mine like the highest spots and being near the window seems important.
Erin, I can't remember where you are located so I don't know what your winters are like. When mine roost in the winter on my tree limbs or 2x4 on edge, they squat down and fluff up their feathers. Their feet disappear among their feathers. Centrarchid had some good photos showing that. My winters seldom get much colder than a few degrees below zero Fahrenheit so I don't have the extreme cold some people have, but I'm not at all concerned about their feet getting cold. If I lived where it regularly gets 40 below I might feel differently, but I have not seen any problems at all from cold feet.
I do believe it is important to use wood for roosts, metal or plastic are good heat conductors and can lead to frostbite, wood is a good insulator. And if you use a 2x4 or something else sawn, I suggest you round off the corners. Just a few hits with sandpaper should do it. That's not mainly to give them a round edge to grip but is more to avoid splinters.
I personally don't care which you use or how you position them. They all work. You'll find supporters and critics no matter what you do. But that criticism will not come from your chickens.