From the prospective of someone who used to work at Red Hat (and familiar with how they like their branding portrayed) my answer to the redhat rhel question is that, the former is and really always was, just a reference to the company name, Red Hat Linux, was always Red Hat Linux, and Red Hat Enterprise Linux, has always being Red Hat Enterprise Linux.
While technically "RHEL" isn't a good reference to Red Hat Enterprise Linux (as I recall, RHEL isn't actually trademarked by Red Hat, which is why it is never used formally - this may have changed though), red-hat-enterprise-linux is too long of a tag, so I think rhel is fine to use on Unix & Linux as a reference to Versions 2.1, 3, 4, 5 & 6.
I don't see much point for redhat as a result, I don't even think it should be a synonym for rhel really, it'd be better (and I'm not sure if this solution exists on Stack Exchange) if the use of the tag would prompt something to the effect of: "This tag is vague, please choose out of rhel redhat-linux (for pre-RHEL releases) or ...".
centos should remain as is, there is enough real separation from the two projects that the tags should be different.
yum update issues etc, will have completely different causes due to different update mechanisms (RHN, yum repository etc).
As for suse while I'm not an expert on the SuSE Linux/OpenSuSE/Novell branding side of things, my understanding is Novell's enterprise, flagship product, is "SuSE Linux Enterprise" I think it's fair to either continue using suse, and keep the opensuse tag as is, as well. It would just be a case of checking to make sure people don't use the SuSE tag where the OpenSuSE tag is more appropriate/correct.