I'm now in Italy and found that a clerk sometimes use buongiorno when I entered the bar. I always use ciao when I greet, but I wonder whether it is OK to greet by ciao or should use buongiorno when I first get a greeting.
Also, should I use it apart according to the situation, for example depending on the age of the clerk and me, the type of shop (cheap bar, more upper-scale restaurants, or luxury brand, etc...), etc...?