I don't often do against the grain on my upper lip - it's a bit disconcerting even though there haven't been any mishaps yet - but elsewhere is fair game if I have the time and inclination. It seems to be most useful under my chin, where it's an easier action than shaving WTG actually. I agree with the people who say that it isn't really necessary though because a couple of passes approximately with and across the grain leave me very satisfactorily shaven in the usual run of things.