The only Jeep I have ever seen them on (with my own eyes in person) was when I did the RHD
I think the front carriage bolts is a good idea, as it would be smoother, but if you have the light tabs as well as pictured ^^, you would have 2 style bolts now on the bumper... I guess it depends on what look is trying to be achieved.
This is definitely a one off case here as the bumper and then brush-guard, tow hooks, etc. are all jammed in there.
If you had just a plain stock black bumper, you would have the mounting bolts for it more visible and those are hex as well.
So, I said a lot here, but I don't have and overwhelming opinion either way... losing a bolt at the top and bottom to clean it up is fine, carriage or hex... not sure.
cb