Well, if you had a stiff system, it would work. But I can think of a number of applications from my own experience where 4:1 wouldn't work - so for a general rule of thumb (which is all it is), I think 10:1 is a bit much. As I say, its really the natural frequency of the system that matters. Also, for many applications, you also have to consider the absolute acceleration capability of the system. So I'll admit that rules of thumb are probably not very advisable here - its too complicated for that.