The current style we've been going for is that the generic tag is for questions that span editions (mostly meta and settings questions), while edition specific tags are used for edition-specific questions (usually without the umbrella tag).
There are exceptions to this, however. Lately I've seen some retagging of questions tagged with deathwatch (for example) getting the parent wfrp tag added.
In general, I'm not a fan of using the generic tag to mean a specific version. I see a few issues with it:
It requires a retagging when a new edition comes out (as is about to happen for Dungeons and Dragons).
The assumed highest edition can be murky. Is Saga edition the latest edition of the d20 Star Wars franchise (same publisher, same licenses, entirely different system)? Or a new franchise on its own? What about games that have crossed publishers? Does West End Games's D6 Star Wars system count as part of the same franchise as WotC's system, or its own? What if the most recent printing of a product is a reprint of a legacy system (I believe both D&D and WoD are going to be in this situation soon)?
How do meta and settings questions get tagged under this system?
Now, there is a caveat that's pretty important. Some systems don't have the volume to support a full tag structure. In that case, tag them with the lowest level that can be supported (in addition to more specific tags) and include edition detail in the body of the question.