I asked two questions about Diablo 3 yesterday. One user, in the comments, felt I should throw beta on them since Diablo 3 is still in beta. I'd considered it when writing the questions, but wasn't sure it would be necessary since the questions were about gameplay elements I felt had a very low likelihood of changing by release. But maybe I'm wrong. The tag wiki for beta is empty, so there's no help there.
There are a few approaches I could see for the beta tag:
Add it to any game in beta, no matter what. The downside is that when the game releases, a lot of questions might be tagged with beta which are completely valid for the non-beta game. Maybe not a big deal.
Don't even bother with beta. In some ways it seems roughly equivalent to having a version-number-x tag added to every question so that you're making sure everyone knows what version of the software you're asking about, in case it later changes. That would be awful. Of course, beta versions are more likely to change than released versions. Which in turn means even if the question is marked beta, if it was about an in-flux system, the question might already be obsolete even during beta due to development changes.
Add beta only for questions where you suspect there is a high likelihood the system may change by release. Of course, this means you're guessing, and is often subject to the same problem as #1.
Your solution here.
Obviously, based on lack of tagging my questions with beta yesterday, I'm leaning towards #2 or #3, but that might also be because my questions dealt with things that I feel are unlikely to change, and thus I didn't have to worry about them becoming obsolete as much as other questions might.