Nothing wrong with round or rounded roosts, lots of people use them. Chickens can curl their toes to an extent, but do like to rest their body on their feet. In cold climates their feet get the most warmth from the flat side of a 2x4 or even 2x6. A really big roo may even want a larger surface to sleep on, like a shelf. I have one that sleeps on top of the nesting box, which is flat and linoleum. No hassle to scrape the poop off it in the AM. People use closet poles, too, but IMO they are not thick enough. I like those 4" or 5" tree branches cut in half, if it's not terribly cold there. Actually it is often recommended to round the edge a bit if you use a 2x4.
Originally I used the narrow side of a 2x4, before I found BYC. When I read about using the wide side, I changed half the roosts. They went right for the wide ones.