For what it's worth, when I had a rooster with bumble foot, the vet strongly suggested that I set up roosts of varying widths, so each chicken can find its "comfort zone". I currently have 2x4s wide-side up, but will be adding some that are turned on end, and some with the corners sanded down further, to give them that variety. Apparently flat feet when roosting can sometimes interfere with blood flow in a big bird with foot issues...

He did go fabric shopping for the nesting box curtains last weekend!
I've been wondering about the same thing you asked about the roosting bars. We plan to use 2x4's and route the square edges so that they are rounded. But...now that I read your thought on a flat board...I might do both...you know give them a choice...some of them might prefer one or the other.