I've met a few people who absolutely demand you call them dr, all of them have been pricks. One dude loves the confederacy and teaches at my old high school, I made it a point then and I make it a point now to use his first name. Glad I never took his class I probably would've failed after saying my ancestors killed yours.

Also something I've noticed in college is that professors who don't require you to use dr. are usually much nicer and more caring