Yes I do, I think everyone I know does this, but I can't speak for them. These days I shop a lot online but when I wanted to see what the windows 8 "load" was I went over to Fry's opened up explorer on their laptop and right clicked on Properties to see how much space was available vs equipped. (I ended up getting a Win7P laptop but I had done that too with Win7). Opening up the 'software' center in control panel you can scroll down and see how much space each package is using. I also took my current Debian/Ubuntu mashup distro and ran through the apt database to see how much I would need in a second partition to co-boot it and windows, and then compared prices for the stuff equipped with my required size drive (given that) and the smallest available drive. Ended up buying the smallest available (120GB) pulling it and putting in a 500GB drive, and selling the 120GB drive for a net cost about 10% more than the cheaper drive.

Now I certainly don't expect anyone else to be that thorough, but my wife always researches available space and was very disappointed in the Nexus 7 which hit her price point at 8GB but didn't leave enough space for her stuff (and of course no SD card slot which finesses the discussion).

But we can anecdote all we want and not make a lot of progress here. If you believe that you should be able to use every byte of space that is advertised on the box, I'm not going to be able to dissuade you. If you accept that the "actual" space is going to be less than the "advertised" space how do you gauge the risk when you buy? Do you just make assumptions about how much of the advertised space will be available? Do you have some sort of internal metric? 90% good, 10% bad or something like that? Do you ever check that metric pre-purchase?