To me, the best method is using qmail-scanner, but that has the side effect that you can't toggle on/off spam and virus filtering on a per-user or per-domain basis. As long as you can live with that (I can) then I'd go with qmail-scanner. :)
That's not entirely true, about not being able to toggle on/off spam filtering... you can configure spamassassin to whitelist mail to specific users and domains, but each time you change it MAY require a restart of spamd (it probably reads local.cf only on startup).
In particular, the "whitelist_to" and associated configuration options. They accept wildcards, so if you wanted to not spam filter any mail to the domain winworld.cc, you could just add "whitelist_to [EMAIL PROTECTED]"
I haven't actually tried this, so we're going on theory entirely here.
Admittedly, this "solution" is ENTIRELY off topic for vchkpw =)