[squid-users] antivirus with squid

I searched in the mail list archives for the configuration
of AV viruswall trend micro with squid, but I have some doubt:

Case 1)
        clients --> hiearchi proxy --> virus-wall --> internet
        All the traffic to the internet go through to the virus wall.

Case 2)
       Equal to case 1, but the hierarchi proxy have configured
       cache_peer access for the virus wall in the case of \.exe$
       \.vbs$ \.zip$ ..... were found in url. In this form not all
       the traffic go through virus wall, only suspicious archives.

Wich it is the best configuration?
How does this affect the performance?

Thanks in advance
Emilio