Exchange 2007 SP3 schema upgrade – which domain and command should I use?

Hello Experts

I'm going to upgrade our Exchange 2007 SP1 environment to Exchange 2007 SP3, but I'm a bit confused about how to do the schema upgrade.

Our domain-setup is as follow, except that I have replaced our company name with contoso.
 
contoso.local (root domain)
edu.contoso.local (education domain)
emp.contoso.local (employers domain)
shared.contoso.local (shared services)

Our important servers in this scenario are:

ROOT:
dc01.contoso.local (RID, PDC, Infrastructure, SCHEMA, DOMAIN NAMING)
------------------
EDU:
dc02.edu.contoso.local (RID, PDC, Infrastructure)
------------------
EMP:
dc03.emp.contoso.local (RID, PDC, Infrastructure)
------------------
SHARED:
dc04.shared.contoso.local (RID, PDC, Infrastructure)
cas01.shared.contoso.local
hub01.shared.contoso.local
mxb01.shared.contoso.local

As you can see, our Exchange servers are located in shared.contoso.local, and hosts mailboxes for the root and the employer's domain.

My question is now how do I upgrade the scheme from Exchange SP1 to SP3?

I have read a lot of articles, and I can see the best way to control it is to run:
setup.com /prepareschema – from the CAS-server. But do I have to use the /DomainController parameter and point it to the dc01.contoso.local (the schema-master)?

And what about the /PrepareAllDomains parameter? Do I have to use this to prepare all my domains?

I know this is a very long and confusion questions, and my grammar and spelling is not making the understanding better, but I really hope that there is a kind expert somewhere out there, who will help me by answering my questions.