There's definitely several different opinions on this point. I was told flat was better by someone who had trouble with rats or mice nibbling on the birds exposed claws while they slept on two inch round roost.
My roosting bar runs down one whole side of my tractor, and is a five quarter by six Inch (apparently that's lumber code for a true one inch thick deck board, that's really one inch by 5.5 inches).
It's a long span (16 feet), so I turned a salt treated 16 foot, 2x6 short side up, and put the 16 foot deck board over it. The 2x6 keeps the deck board from sagging or wobbling. I liked the deck board edges were already rounded, it only took a few extra minutes to also sand them a little smoother.
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On the left you see the deck board on the 2x6. At the other end of the tractor is another deck board, but also a couple of 2x4's, short side up, sanded, and finally you see 2 inch PVC schedule 40 pipe covering the scaffold bracing that keeps the frame of the tractor rigid.
I put a bolt through the PVC pipe and bracing to keep it from spinning, and wrapped it in Gorilla tape to make it warmer, and to give them more bite on the surface when they perch on it, which they do during the day.
For what it's worth, the birds all roost on the 5.5 inch wide deck board, in the left corner, closest to the door.
I went this way to avoid rat or mice problems. How real that concern is, I don't know.
Having made that choice, here's the advantages I can see. No mice bites, they can cover and keep their claws warm, with a metal six inch drywall knife, a quick walk down the perch and poop is gone, and finally, it really does add practical and useful space the birds use. They run up and down the board all day long and easily pass each other, that passing ability also helps when they settle in to roost, they can work out, who's next to who better.
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As much as we agonize over this issue, I'm not at all sure the birds care, one way or the other, they're pretty versatile.
