When To Stop Writing

So many writing advice gurus advocate plugging away, getting words on paper, working through a "block." I wonder if that's totally the right thing to do? Doesn't it depend on what a person is writing? If one is writing a book on the life cycle of grub worms, then I can see value in plugging on. If one is writing comedy, well, maybe pushing it is okay. But if one is writing serious stuff where every word on paper matters and should have a gem-like quality, I wonder if the best course of action isn't to stop and put it away the minute mental fatigue sets in. Is something--anything--on paper good when next session you have to scrap and rewrite it?