While Pactra paints were still around, that was pretty common advice at my LHS. I don't recall exactly what the circumstances were (bad batches, old stock, just plain nasty brand, etc.), but you would occasionally find debris floating around in your paint, like large clumps of congealed pigments that wouldn't break down in thinner or large particles of paint that had set up and cured in the jar. At the time, it was sound advice because any of that stuff would ruin an airbrush session rather quickly.