I don't do a lot of modeling - my architectural models tend to be mostly for demonstration of some lighting discussion. Having said that, my experience has been that I can create minimal polygons and not even weld them and get away with it. For purposes of walls, frames, and doors, I don't even bother with the beveling because you can't see those small rounded corners unless you do a closeup.

In this setup, which I assembled to look similar to yours, none of the boxes are welded - no two polygons, whether in the same box or different boxes, are connected to any others. Since they are not welded, there is no danger of smoothing accidents, if I happen to turn on smoothing. However, this does not work too well if I enable subdivision, but really I have no reason to subdivide flat boxes, so it's not an issue.

Here's a render

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)