Its always better to customize the browser after installing to control what sort of data it stores, leaving browser with default installed options might put your personal information under risk which is just what warned by security expert on autofill web forms bug found in Safari 4 and 5 versions.
Jeremiah Grossman founder of WhiteHat security mentioned in his blog post that the autofill web form bug in Safari could allow malicious website to uncover first name ,last name,work place,city, email address details of a user automatically even that user never visited that site or entered his personal information rather for that matter.
If you are a Safari 4 or 5 version user just hit the settings button in safari and click preferences and select “AutoFill” button you will observe Safari automatically fills web forms using info from your address book card.
If you have stored all personal details and information on address book card when you visit a malicious website , that website can access all your address book card data from safari and send to hacker invisibly.
Safari users using 4 & 5 version need to disable autofill web forms option immediately to protect their privacy and from loss of their personal information.
Apple has been notified about this vulnerability by Jeremiah Grossman on June 17 th 2010 but got no response from Apple so he revealed the autofill web forms vulnerability in Safari public.