Varnish helps to some extend to compensate the dry-down effect. Many people don't like it, because it alters the matte paper surface, but it helps to bring out shadow tones. The question, by the way, is not only one of becoming darker as such, as the dry print effectively looses contrast in the shadow, and the darkest zone, which appeared with luminous shadow detail in the wet print, tends to become obscured. This is what the printer in my opinion primarily has to keep in mind: the issue is not exposure time alone, but also contrast.