I learned from my first outdoor project that plywood doors painted on
one side will warp when it rains.

But what about indoor bookcases in a climate where the humidity is
moderate most of the time (SF Bay Area)? This installation is three
built-in bays, sitting on a base and screwed to a wood wall. The outside
surfaces and edges of the boxes will be hidden by trim. I don't want to
paint more than I have to but I don't want warped cases in a few years.

Thanks in advance for advice.