I don't know if it is documented, but it would be a good practice to assign all the disks on a shelf to a single controller. Of course, best, best practice is for each to have their own stack. But we all know how things work in the real world.
What we have done in the past in scenarios where you start with one or two shelves, is to split the disk assignment between the heads. Once more shelves are added, the new disks can be re-assigned so that each controller owns all the disk in their "own shelves". You can do this via the disk replace commands (if you have enough spares, and the patience). It is also a good practice to spread the disks in the raid groups in the aggregate over all shelves that the controller is using.
So for 6 disk shelves, maybe shelves 1, 2 and 3 belong to head 1 and shelves 4, 5 and 6 belong to head 2, and then in the aggregate for head 1 the disks would be added such that the first disks from shelf 1, shelf 2 and shelf 3 are added first, then the second disks from all three shelves and so on so that IO to each raid group is spread across all shelves.