My goal would be that in the list below, the first option would not be true:

That is, just because a teammate added me as a reviewer, it does not mean I wish to immediately start getting email notifications of all the activity. I prefer to review Pull Requests on my own schedule (e.g. beginning and end of day), so it's a lot of noise mid-day. Plus if a reviewer is out of the office for a few days, it creates unnecessary emails. Just one email about the open of the Pull Request would be ideal (or other major PR state changes).

The main reasoning is that I think only users themselves should be in control of when to start receiving lots of notifications. This is the one scenario (from the above list) where someone else appears to be in control.