My colleague (who teaches painting and decorating at our college) says that indeed, putting a finish on just one side of lower-grade materials can mean it’ll warp on that side.
Think of what happens when you wet one side of a piece of paper, it tends to curl to that side as it dries.
He says the term is ‘Back-priming’ and ideally, you will finish/paint all sides of your skirting before fitting…
never heard of that in 27 years as a decorator,maybe i’m just a rough arse