I just asked a question about Windows 10, and I noticed there isn't a tag yet. Could it be worth setting one up now, so it's in the correct format etc?
I'd say it's time for the tag when there is an actual Windows 10 version available that people can get their hands on.
Similar to our process for the Windows 8 Preview, we'd first start with a dedicated tag for this early version, and only once the real one is out in the wild, use an official tag. We'd then check whether questions that were posted with the preview tag are also applicable to the final version, and retag if necessary.
At this point in time, I don't see any on-topic question carrying an "official" Windows 10 tag, sorry.
Looks like both the tags were created anyway and in the meantime people have gotten a hold of the preview.
I've gone ahead and synonymised windows-10 with windows-10-preview. As with Windows 7 and 8 features can and do change before official release so once it is officially RTM'ed then the synonym can be split.