Except for the scalehouse and other small structures in the foreground, none of the structures in this photo have modelled backs. There is a back, or, in the case of the larger buildings, a lot of heavy-duty bracing, but it's simply blank .060" styrene.

Her's the same area from the opposite direction:

This one, built from a bag of broken parts from the used table at my LHS, has only two finished walls, as that was all for which there was parts. The other walls are more .060" styrene:

The brick building shown here has only its front finished. The other walls are painted a "brick colour", but they're barely visible from any direction:

I built four of these coal dealer's storage bins. Three are finished like this one: both ends and the side facing the track:

The fourth has both ends and the truck loading side finished:

The two kit-built structures on the right are fully modelled (I didn't feel that it was worth the effort to replace the rear walls, even though they're not viewable), while the two kit-bashed ones at left are three-sided:

As is this scratch-built icehouse:

And the GERN complex is modelled only on its visible sides. Even the free-standing section, at right, has only two finished sides:

I do, however, paint both sides of both rails, as I often take photos from the layout. Here's a shot of the area shown in the first picture:
...and a view on the layout (the track has yet to be ballasted) from the same direction as my second photo:

Another reverse-angle view:

This shot looks right across the aisle to the area of the scene above:

And a view along the tracks (I think that's the northern lights overhead, but I don't like the looks of those clouds in the distance
):

Near the icehouse:

And a view from staging:

The buildings in the foreground at right and centre are fully modelled, while the one at left has a finished wall (facing the camera) cut-off on an angle at the edge of the layout, with the cut area simply blank styrene. The front, facing the building at centre, is also finished, as is the unseen wall, which faces the five-storey building to the rear. That wall can be seen at right in the photo above.
For an around-the-walls type of layout, using finished walls only where they'll be visible allows you to double the size of many structures simply by using the unseen wall on the side where it will be seen - many of the structures above are examples of this.
Wayne