The keyword filters are applied to a mailbox so all of the addresses pointing to that mailbox will be affected. If each of the addresses have their own separate mailboxes then you will have to set up the filters on each one.
I would definitely recommend using the junkmail filter instead, though. It is smart enough to not catch most legitimate commercial email. Trying to keep up with spammers using keywords will most likely make you crazy eventually.
- DreamHost Head Honcho/Founder