I tend to cringe a little when someone says they're painting white on a figure. White is such a strong color and to be honest, its not really all that natural of a color. Especially in scale. Its kind of like painting pure black on a model.
I think that anything needing white, should be given an off-white coat .... even if its just a touch of off-white. After that, I think that shading and highlighting will do the rest. However, if something is painted pure white, there's not much that you can highlight it with. I think long-story short ... that's what I was trying to get out.
Just my 2 cents.