Height of uprights for "twin slot" wall mounted shelving
The concern is mostly pullout strength, and not shear. The bulk of the leverage of the shelf pulling away from the wall is above the shelf. Therefore, screws above the shelf carry most of the pullout load. It's unlikely that you will have insufficient shear strength if you use 3-4 screws per standard. 

My point is that you probably won't need the standards to extend downward from the shelf, but some length upward is a good idea.