There is a common misunderstanding about how seeding and exchange replication happens for a mailbox database copy. As an administrator, you can choose the source from which the seeding or re-seeding of another (or new) mailbox database copy should occur. This information becomes very important if you are involved in Exchange design work, that too using multiple datacenters where the WAN bandwidth becomes very crucial.
A good feature of Exchange 2010 is that the initial seeding or re-seeding of a mailbox database copy can be done either from an active copy or any passive copy of the database. It is recommended to use the passive copy to reduce the load on the active one. But, once the database is seeded, source for the continuous replication will always be the active copy of the database.
The wizard for updating the database copy makes it clear.
In short, seed or re-seed from any copy of your choice, but continuous replication will always be from the active copy of the database.