Zig packages are pinned to a commit hash. When you fetch them, you can use tags or branch references to fetch them, but then those are resolved to a specific hash and saved.
It doesn’t matter what the tag is named. Either is fine. Or you can use your own.
There’s definitely no requirement, and I haven’t noticed a convention. My personal favourite is v1.2.3, the Zig compiler repo uses 1.2.3. Do whatever feels best to you, it’s your repo and I doubt Zig will ever enforce a specific format.