There's no UI for this obviously, but it may be controlled by a hidden preference in Safari's plist file. If you have Apple's Xcode installed, it comes with a tool called Property List Editor that lets you graphically view and modify plists. This tool is the way I recommend for discovering hidden preferences in programs. In the event you do NOT have this tool, I can investigate this for you - as I have the tool installed, as well as Safari 4.0.5.