I think the thumb rule of 2xrated voltage for a capacitor is linked to the technology of the capacitor. Chemical capacitor are more sensitive to voltage than plastic or ceramic capacitor and are easily destructed by quick overvoltage pulse.
the second pint is the lifetime : More voltage means an increase in the working temperature, and a decrease of the lifetime of the component....
In my mind, for electrochemical capacitor, we should keep the x2 factor, but we can use the rated voltage for the ceramic ones...