/System/Library/CoreServices/Archive Utility does have a preference setting for the archive format, but it only seems to control archives created by Archive Utility, not with the Finder. Plus, its format choices are Zip, regular (cpio archive), and compressed (gzipped cpio) -- tar and tgz are not on the list.
It's not quite what you're looking for, but Snow Leopard improved the "service" system to the point where it's actually useful for things like this. Run Automator (I know, but stick with me...), create a Service rather than a Workflow, set "Service receives selected" to "Files and Folders", then copy in the second action from the workflow 11 pointed you at, and save it under some reasonable name. Then select a file in the Finder, right-click, and look at the bottom of the contextual menu for the Service you just created. It'll also be available under the Action (gear) menu in the Finder toolbar, and under the Finder > Services menu. You can make a keyboard shortcut for it in System Preferences > Keyboard > Keyboard Shortcuts > Services (then find your service, click in the invisible right column, and type the shortcut you want).