I wholeheartedly support this, even though the suggestion seems to lead to screams of 'unfair' and 'fun' whenever I see it mentioned. As necessary as scumming is for a beta, I shouldn't have to feel like I'm cheating when I accidentally dig out a bit of wall when my hand slipped designating. Not that the process shouldn't be preposterously cost prohibitive, so you really do feel stupid when it happens. That SHOULD be the case.
The whole idea of hiding tiles seems like a band-aid that the lighting arc could fix. If an area is totally unreachable, I shouldn't be able to see what is going on, and aside from stationary discoverables, the area should be darkened/blacked-out.
I also think that the same should be considered for floors. I detest roads (and will probably never get a king without striking fun), because they look like ugly water that gets all flashy when I dump something on it. Floors serve the same purpose in my mind, except I can build floor in midair. A floor built on ground should be noticeably cheaper stonewise than building a floor up in the sky, so that I can put stone under the feet of my military in the barracks without instigating a hauling APB, or to reasonably 'floor' roads. And then the roads option slowly disappears, never to be spoken again, except by dwarves in the fellest of moods. Or next to elf chunks in the HFS.