Well, the tags are exposed via the rest api, so you could probably use w/e version you wanted. Just need to construct the rest request.

But the SDKs are backwards compatible.  So if you use 6.5, you can target a 6.0 vCenter no issues.  Just don't try to access any new 6.5 features against 6.0.  You can use a version check if you need to do so across multiple versions of vCenter.