I have a Java webstart application, fully signed, and it has been deployed live for over a year now. We have never been able to get it to work with Safari for some reason.
Due to the limitations mentioned here, I've avoided the built-in AppletViewer of Java Web Start. As an alternative, consider the hybrid applet/application approach shown here, which works in Safari, et al. Note that the request for access to your computer can be safely denied, as the applet runs correctly in the sandbox.