The SA will not ignore rewrite for publicly available websites provided that you have a rewrite policy for the publicly available URL's under resource policies-->web rewriting policies applied to the user roles you are assigned
Regards,
Jay
If you do wish to access public websites through the SA, remember that - in addition to their being included in a rewriting policy - they must also be included in a web proxy policy. The SA will not access the a website through the external interface.
Ken