Hi Jon,
One vote for the mantle piece.

I'm one of those people who are bummed when a perfect part gets damaged (I realize many couldn't care less) and my MR tank sustains wear and/or minor damage every time I ride. If I had put a NOS tank on it, it would now have substantial knee wear and the standard triple-clamp dent(s), along with several branch gouges and who knows what else.
I've got a NOS tank for my RM, and my OEM CR tank looks pretty much perfect on the bike (there's a tiny dimple you'd never find unless I told you where to look, and some seat wear that's only visible with it off). Both these tanks are packed away, and the Hondas have Clarke aftermarket plastic tanks. With these, I have zero worries and am free to concentrate on riding, not damage control. The RM rarely gets ridden these days, and still has the original tank on it.
If it were 1978 I would say throw the NOS tank on - you can always order another one - but nowadays those tanks are really scarce, and getting more so by the day.
Just my $.02.
Ray