Ok, Let me summerize.
The fifth dan test qualifies you to enter the shidoshikai and become a full instructor. People lower than fifth can still teach as an assistant instructor (Shidoshi-ho) and enter the shidoshikai under another teacher. You only have to enter the shidoshikai if you want to teach as a Bujinkan instructor. I know of at least two guys in Japan that are above fifth and do not want to teach and so they never entered the shidoshikai.
To train in the Bujinkan, you need to be a member according to the rules. There is a card issued every year. I know of a few teachers that will let this slide for a few months when people first enter. If three months down the line they decide Bujinkan is not for them, they can freely leave. From three months you need a card and a uniform. But if you want to train at the honbu you need the card. Most Japanese shihan I know will not teach a person who claims to have rank in the Bujinkan but no card.
Ranks certificates from the honbu are optional. But if you do not get the certificate and registration, you are not recognized as having rank by Japan. If you go from one state to another and start training in another dojo, the rank from Japan should be enough to keep your rank and train at that level. Anyone who refuses to recognize a rank certificate signed by Hatsumi and tries to make you train at a lower rank in a BUJINKAN dojo is, well..... (And yes there is a story I can tell but it would distract from this thread.)
Kyu ranks rarely get certificates. Their ranks change every few months and it takes a long time to get the things from Japan. Most teachers I know get their paperwork done once a year or so when they come to Japan. Technopunk, I would not sweat the certificate at the kyu level unless you are planning on moving to a new area and dojo before shodan.
