A good article by Michael Cobb about risk of caching by browser :

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Both browsers will cache a document requested over an HTTP connection that has the “no-cache” directive set. Mozilla will not cache any pages by default over an HTTPS connection, whereas Internet Explorer will, unless the user has enabled the “Do not save encrypted pages to disk” option. A Mozilla browser never stores documents set with the “no-store” directive, but Internet Explorer only fully follows this directive when the page is requested over an HTTPS connection. So the only way of ensuring that your sensitive documents and pages are not cached without requiring your users to manually set any IE options is to use the “no-store” directive over an HTTPS connection. All browsers supporting HTTP 1.1 will support this directive.

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