If you want white, get white. Just because you can blow out grey to white doesn't mean it's a good idea. Specifically, if you zap it with another two stops or whatever, there will be tons of uncontrolled spill bouncing around everywhere - it's bad enough with white.
Knocking back grey to black is better, but even so, if you want black, get black.
Grey is good for moderating a bit lighter/darker, and nice to be able to put a spot of light or white in the middle. But not for changing the tone completely as a regular thing.