I haven't done anything to the engine casings yet and I would agree with you that a glossy finish there would be wrong. If I ever get round to it I would like to have bare vapour blasted ally on the engine casings.
That worked fine on a couple of my old Brits, and is quite resistant to oil and road muck staining, but I was always pretty quick with the soap and water if I came anywhere near any Salt!
I used the VHT Gloss lacquer on the outer covers, over Gold, which I think gives it an extra something, for both looks and protection.
There was another reason that I lacquered them. I polished various areas of the covers before painting, and carefully wiped the wet paint off the raised polished bits. (The circles round the inspection windows and the Ducati name) So the lacquer helps protect the bits that would otherwise have been bare.
Personally I think it looks more "all of a piece" if the belt covers and outer engine covers are all the same colour, but we're all different, which is why I spend a fair bit of time ogling pictures of other folks Monsters..
The Gold on mine was intended as a nod to the Matchless G50 and AJS 7R by the way.