Hi!
Greetings can be as simple as just saying hello, without mentioning any user and still work positively as the author of this post stated. But when it comes to avoiding mention-spamming in comments under someone posts, I think mostly the long-time Steemians with high reps and much SteemPower don't appreciate it. I think that's because they are confronted so many times with people trying to grab their attention that it becomes annoying.
The more minnow-type Steemians would probably have a preference for their username typed and mentioned in comments.
One of the keys to effective communication, is to make sure you understand who you are talking to and maybe adjust your message to be helpful, valuable and contributing without being annoying at the same.
I would tell you a story, but this comment is already way too long :)
There's no fixed rules, but I would advise people to try to not be annoying. If you waste the time of a whale or other experienced user you may just get muted (or worse).