You can add and edit tags on the go. I personally prefer submitting tags beforehand, at least those that describe and give info about the story. If I find one of the tags feeling spolierish, I just don't add it. That being said, you can't always predict your new ideas that arrive during writing, so when something new happens, you can casually drop a tag about it if you find that thing that happened actually valuable and not just a one time surprise.
Similar think with genres. I try to pick the ones best describing the story in the whole, even if we currently have only like 2-3 chapters.
You don't have to feel pressured about either having not enough or too many tags, just add as you see them fit. Although, adding every single tag that is somehow linked to something that happened in the novel might be too much.