I feel you about Holmes over Sherlock! Generally I go with whatever the narration / in-game text of a canon uses, and if that's absent I feel presumptuous and overly friendly (to a fictional character...) if I use a first name that barely any of the characters do. I also like to collapse names down (Hokuto>hkt, Martin>mr, etc, as long as it's clear in context) which is something that grew out of not wanting to show up in keyword searches on Twitter.