It is extremely rare for Linux software to require a particular distribution. I've seen a lot of questions with the ubuntu tag and retagged them to linux, because the questions were not about Ubuntu, they were about Linux.
If the question is “I want software that does X and runs on Linux, and by the way I'm using Ubuntu”, then Ubuntu is not a key requirement, because any Linux software will run on Ubuntu. “Available as an Ubuntu package” would be a plus, but that doesn't warrant a tag.
The ubuntu tag should be used in addition to linux only if the question does not apply to every Linux distribution. For example it would make sense to use ubuntu if integration in Unity was a requirement.
The ubuntu tag should almost never be used instead of linux. That would only make sense if a question was absolutely specific to Ubuntu, for example an application to interact with Launchpad.
Most Linux distributions will not warrant a tag at all. Ubuntu does (and even then it's overused) because it provides an integrated environment, and it makes sense to have requests that are somewhat specific to Ubuntu in that they require good integration in that environment. centos or debian or arch-linux wouldn't make sense in this way.
See also How specific should the OS tag be? for a more general perspective.
Fedora Extrasor someplace else. This is a problem of RHEL/CentOS, not a problem of the software which is the answer. In those cases I feel that the software recommendations should still include those not packaged (or, say, being packaged on Debian/Ubuntu but too old there), and the asker has the drawback of deciding whether that’s still worth to install them.