I used to think that the rule (sez who?) was that Latin names were anglicized when the root of the name was still in use. That explains why in English we turn Constantinus into "Constantine" and say Hadrian instead of Hadrianus. But this rule breaks down quickly because you never see "Gallien" rather than Galilenus and you absolutely never see Claudius turned into Claude. Extreme anglification would result in some comical transformations like "Antony Pius", "Commode" and, heh, "Tit". Still I see people often still writing names like Domitianus or Maximianus when those suffixes have gone out of style by World War II at least. Altogether we sit on the conservative side between the German purists, who rarely change the old form names, and the Romance languages which feel free to make new names altogether. Nero becomes Nerone in Italian, Aemilian turns to Émilien in French and the absolutely egregious Spanish conversion from Iohannes to "Juan"! Pet peeves anyone?