Some questions apply to functionality present only in (or after) certain versions. Others are for legacy versions such as 5.2.
Tagging it mathematica-x is redundant on a site on Mathematica. Since version 5 is still fairly used by folks (going by questions on SO), I suggest version-5 through version-7 and pre-version-5 for the rest.
I don't see the need for having tags for minor releases and tags for the current & future versions. That might give the impression that the question is applicable only to, say, version 8, and not to 9, which will probably not be true for most cases. I can see the use in creating version-8 when 9 is out and there is a difference between 8 and 9, but not prior to that, merely to differentiate between 8 and previous versions.
I also think that a version specific tag should be added only when it is definitely something that's different from the current version. E.g., "How do I plot Sin[x]? I'm using version 7" should not require version-7.