Procedure for rebooting Domain Controllers?

No need to stagger reboots. Active Directory, being a multi-master system, can sustain any order of domain controller reboots.

As for safety and redundancy, it is nice to have roles spread across different DCs, so consider that for a future project. However, that won't be a consideration for reboots.

EDIT

To address the following comment:

I read somewhere that transferring FSMO Roles, although not necessary, might be a 'defensive move', just in case the server somehow failed to be back up. (Thus, reboot DC02 & DC03 first, transfer the role, then reboot DC01).

I think that it makes sense from a strictly theoretical point of view. A domain controller with all FSMO roles that goes sneakers up is a horrifying thing to endure. However, in practice, I see two problems:

1. A server should not be thought of like a tumbler of nitro glycerine on the back of an epileptic lemur. If preparing to reboot your server causes dizziness, gives you a dry mouth, and otherwise makes you want to start abusing household cleaning products, then the solution is not to start moving FSMO roles, but rather to get your server into a state of health that makes you comfortable with rebooting it. Well managed servers are robust things that can be rebooted with impugnity.
2. Moving FSMO roles, while as technically simple as a few mouse clicks in an MMC, isn't such a lighthearted thing that would cause one to play a rousing game of "FSMO, FSMO, who's got the FSMO!" for every Patch Tuesday (which coincidentally precedes "Career Change Wednesday").

In short, don't move FSMO roles except to practice good systems engineering tenets. Unless your server room is full of lemurs. In which case, hey... lemurs!