When we first got WF, we had to tag people manually. At some point, that changed and we no longer had to do that. In the last few days, it seems we need to got back to manually tagging because people were not getting emails.
Hi - I recommend to my users to always tag people just to be safe. (Obviously, just the first status update, for replies, they only need to do new people to the thread).
There are some email notifications that go out automatically based on certain criteria but we all get confused about which automatically triggers what and/or the user has turned off that email notification anyways so doesn't get it.