I am one of those instructors who is more formal. I do expect and hope to get coherent email communications from students. And, I do expect that email communication is treated with the same respect as a face to face visit during my office hours. Thus, I am always a little surprised (maybe frustrated), when I get incoherent emails from students that are written like a text between friends or emails that begin with my first name and are very casual.
No where on my course syllabus do I list my first name. I sign all of my emails and Moodle messages with my title of choice: Prof. A. I even explain during the first lecture that students can refer to me as Prof. A, Dr. A or use my entire last name. There are not invitations to mispronouncing my name! I don’t even go there. This post is part rant and reminder–until an instructor says you can call me by first name–always use the professional title.
Dr. Janni Aragon 
I was at a conference for astronomy teachers this Summer. The last session of the conference was an “open mic” where all 150-200 of us gathered together to share our trade secrets and ask for advice.
The topic of “what do the students call you?” came up. One prominent figure in the room said, “I cannot fathom my students calling me anything other than Dr. Fxxxxxx.” The room erupted! We soon discovered a very, and I mean v-eeeee-ry wide spectrum of names we give ourselves. On one end, those like Dr. Fxxxxxx. Somewhere in the middle, first names. Towards the other end, a nickname. Off the far end of the spectrum, one fellow (with a name that’s easily turned into a fart joke) actually asks his students to call him Dr. Crazy because it’s a name he chose and it’s better than what they would come up with on their own.
Me? I’m happy with “Peter” in the classroom. I sign my email with “See you in class, Peter.” The only one I won’t accept (well, in addition to “hey you”) is “Dr. Peter” In my mind, that’s reserved for Dr. Peter Jepson-Young.
Dear Peter: I should have spoken to the gender and age components with this. I hear students around me (halls and in my office hours) refer to male colleagues by their professional titles and then to female colleagues by their first names. I do think that some of the students assume a familiarity with female instructors in comparison to male instructors and this plays into collegiality.