Yeah I think self-policing will be required and in fairness I think collectively we're good at that.
I did think of how you'd cover mechanics because there could be spoilers with that (particularly certain slates if not yet revealed) but some people may think a post explaining how it's been used is a spoiler, others not.
I think we'll have to make it clear to act out of caution and if in doubt, use the spoiler tag.
Happy to go with a month and then after that move to a courtesy kind of thing because not everyone will play at the same pace, or even get it at launch
Update
Now that I'm in my hotel room rather than on a train; so shall we say until June 9th (4 weeks) all narrative spoilers in spoiler tags. Following this common sense discretion (e.g. late game story beats etc tagged as courtesy). Gameplay/mechanics use discretion and air on the side of caution but if there's any new items etc that were not revealed pre-launch (or if you cannot recall) then use tags during that time too.
That sound good?