You probably won't use a black pastel very often as it's a very intense, almost selfish color, but in those cases where the dark tint is just not intense enough, a black will give that final touch. Several manufacturers offer an 'intense' or 'serious' black which are ideal.
White will be more useful, especially if you have chosen the second lightest tints of the mid-tone colors for your set. If you are going to be using the white mainly for highlights, consider buying one from Unison, Sennelier, or best of all Schmincke. These tend to be softer and easier to apply to an almost finished pastel painting.
Lastly get a couple of gray pastel sticks. Rather than taking a neutral gray, take a warm (Davy's gray or Mouse Gray) and cold (Payne's gray or Blue gray) color.
