You're not gonna get me to lying? LOL. I won't sit here and pretend to know if there are any benefits or adverse effects of their roost. What I can tell you is they prefer to roost on a rounded bar. As they do in nature.Help us understand why -- is it so they can curl their toes and clutch it? If their feet are flat for roosting, does that potentially produce a higher incidence of bumblefoot? Is the pressure applied to their feetsies different given the flat surface vs. the rounded? Thanks for your input.
Here is a post I made a while back on how I tested what they liked better.
All one really has to do is look at birds around the area. They will prefer to roost on the power lines instead of the tee on the pole. My best guess is that it is more secure for them to be able to grip around a wire(roost) than it is to grip a flat surface. When the wind blows they can definitely hold on better.
If you're in doubt, then I suggest, (to all) find out for yourself and do the same experiment that I did in the post I linked to.