All my more creative (read: hippiesque) profs used to let us call them by their first names regardless of subject taught. Then I used to have non-PhDed instructors who'd insist on being called by their last names or just being "sir".
funny story: all the profs in a grad program GENERALLY let grad students use their 1st name (this is not univeral. at UChicago tradition was that everyone went by Mr/Ms - including students - so it was equal respect for all). but law schools are different - they still use "profesor X." i work for a guy at the law school (who is a JD/Phd) - and i call him by his 1st name. all his law students, call him "professor." it's just ... odd.
All my more creative (read: hippiesque) profs used to let us call them by their first names regardless of subject taught. Then I used to have non-PhDed instructors who'd insist on being called by their last names or just being "sir".