My opinion is that bolt heads were seen on panels only for purposes of cost and expedience. There is only one circumstance in which the bolt heads serve a valuable purpose, and that is additional binding points to hold down a clear plexi or lexan overlay. While that has mechanical benefit, it certainly doesn't do much for aesthetics.
With a few exceptions, carriage bolts were added by operators, rather than installed as original equipment by the manufacturer. More likely than not, they were installed as a repair when the OEM mounting methods gave up the fight after a few years of abuse.
As Fozzy stated, it's mostly about what you like, what you remember, and what you are trying to recreate. Most manufacturers (then and now) try their best to hide hardware when possible, as the result is generally considered more pleasing to the eye unless the hardware is decorative or part of a specific theme. But it is a matter of taste, pure and simple.
For the most part, you should never have to worry about your sticks breaking free if you use the right hardware. You (hopefully) won't be leaving your machine in an unattended 24hr laundr-o-mat to be hammered on by drunken thugs, so the added durability of carriage bolts is probably not a concern here.
RandyT