I just checked in DIY SE (the exact reputation numbers for each privilege sometimes vary, but that is good enough) and:
- Create Tags 300
- Create Tag Synonyms 2,500
- Approve Tag Wiki Edits 5,000
So almost anyone can create tags. I’d consider that ~ Trust Level 2. However,
- I think it is way too easy to create tags in SE, resulting in a lot of near-duplicates and other problems
- The initial communities will be modeled on SE sites and can therefore start off with a list of tags based on those sites
and so Trust Level 3 seems reasonable for MVP.