Classroom vs IRL: Spanish 2nd Person [View all]

Besides being totally rusty because when I speak Spanish my mind is frantically fumbling through a heavy imaginary hardcover English-Spanish dictionary trying to utter a simple sentence. one of the things that has always hung me up is "tu" vs "usted"

Way back when, I was taught that you should only use "tu" when you're familiar with the person you're chatting with, otherwise use "usted."

I already get the part where, when addressing more than one you go with "ustedes" ("vosotros" if you're hanging out in, say, Barcelona or Madrid). But, one-on-one, is it really considered slightly (or full-on) disrespectful to use "tu" with a stranger?

Gracias!