Postfix, amavis, dovecot, and TMDA configuration help

I currently am running a similar configuration as outlined here:

   http://workaround.org/articles/ispmail-etch/

In short: internet -> postfix  -> amavis/spamassassin -> postfix ->
dovecot's deliver agent [sieve] -> virtual user

Since I am managing users' mailboxes "virtually", and since I'm using sieve
for filtering, I'm unsure how to install TMDA appropriately.

I believe the most appropriate place to utilize it would be right before
postfix hands the message back to dovecot's deliver agent, so I am not
generating outbound approval requests for the bulk of the spam.

Suggestions on how to set this up would be greatly appreciated.

-dave