I love this Q! And I can sort-of answer it… I had a friend in Seattle who worked in various small opera companies, and this subject actually came up once. She told me that it was usually decided by both the director and the dying one, and it often depended on whether or not she was singing exactly at the time, how comfortable she was with whatever sort of dramatic demise, and how high the sung notes were at the time of death. Apparently, that had to do with the sustainability of certain notes in the range and the body position. (The higher the note, the lesser the slump, the verticaler the better.) As far as I remember, realism was never mentioned. Or probably even thought of.