I would argue that tagging an Ubuntu-specific question as linux is not really helpful. If the question applies only to Ubuntu and not to other flavours of Linux, it should be tagged ubuntu and have no linux tag. If the question applies to various flavours of Linux, not just Ubuntu, then it should be tagged linux and have no ubuntu tag. True a newbie may not know if his question applies to just his distribution or to all of them, but Google search is pretty good at untangling the hierarchy of things already.