Once in a while, people will ask us what height to place the channel for the female bracket in their shelf - generally, in this regard, we recommend about 1/4" below the top of the shelf (this is because any thinner and it becomes too easy to blow out an edge cutting your channel, and waste an expensive piece of wood or material). However, since not all shelves are the same, there are times where this may not be applicable.
The thing that is important to remember is that the thicker the part of the shelf sitting flush against the installation surface (i.e. the drywall in this example), the better distribution of force from the weight of/on the shelf. A thin sliver of shelf material at the bottom is liable to hyper-focus that pressure, embed into the drywall, and result in a slanted shelf. For that reason, you always want to place the bracket channel as high as can be reasonably achieved in your individual situation.
In the diagram below, the image on the left is an example of a poor installation, likely to cause issues, whereas the option on the right will properly spread the force exerted by objects on the shelf and the weight of the shelf itself across that wider surface.
The other important factor to consider, is that the shelf needed to be tilted downwards and snapped into place at a 15 degree angle to connect the two brackets and thus the shelf to the wall. A bracket placed too low in the shelf means the extra material above could interfere with this process and stop you from mounting your shelf. For this reason, you always want to keep the bracket high up in the shelf.