It depends on the setting, and the premise. The last time I had to be firm about this was with a campaign within the great big meta-campaign, where the characters were a newly formed squad within a city's police force. One of the players was immediately thinking in terms of exotic cross-breeds with bizarre motives, and was unwilling to believe that fairly mundane characters were most appropriate for the premise. But it was that or not play.
The character calls himself "Librus X" and all the players soon realised that he wanted to be addressed as "Mr X." They have never done so, regarding it as pretension on his part, and having grasped the idea that just being people was what the campaign needed.