My take on it... it's a model car first. Even if it's a replica of a 1:1 car, it's still a model. So do whatever you want. If you think it will make you happy to clearcoat, then clearcoat. The shiny model always gets the nod. Unless... it is supposed to be satin, such as a rat rod. Personally, I would not coat over the decals, but I may clear the paint before decal app. That's just me, though.