Do I call my TAs (teaching assistants) by their first name?

What should I call my TAs by? Their first name? Or Mr./Ms. (insert last name here)? 

The beginning of freshmen year, I was super confused by this. Luckily, my peer mentor set me straight. He told me, "My usual rule of thumb is to call teaching assistants/staff members by their first name and faculty by their last name." 


Easy, right?