If I have a question concerning CentOS 6, I would use the common tag centos and maybe also the less commonly used tag centos6. Is it a good idea to add the tag redhat for these questions?
As CentOS claims to be binary compatible with RedHat Enterprise Linux (RHEL) I assume questions about CentOS would also be interesting to RHEL users. But on the other hand if I add the tag redhat, the tagging of my question will be less precise.
RedHat is RedHat.
CentOS is extremely similar to RedHat (bug-for-bug-compatible in many cases), but it is not RedHat.
The most major and glaring difference that comes to mind is that you can't subscribe a CentOS box to RedHat Network.
Your question will get plenty of views with the centos tag. If it's a general Linux question that doesn't need CentOS-specific knowledge you can add the linux tag, a scripting question should get the scripting tag, etc.
I suspect but may of course be completely wrong that, in the context of SF, Red Hat will be used by those wanting having commercial support. CentOS will generally be used by those who don't want or can't afford commercial support. As such I feel it likely that the two audiences will be somewhat different.