Greetings are one situation where cultural differences are most visible. Once I introduced myself to an Asian student, and he surprised me by bowing to me as we were shaking hands. Bowing! To me!! I was... rather flattered.
To learn proper greetings, I take my cue from my environment, learning the proper protocol by following the example of others. I try to do it right, but it's not always easy. Once in the jungle in the Golden Triangle of Thailand, some children in a ditch saw me walking by and did the namaste gesture, accompanied by a slight bow. Wanting to fit in, I reciprocated the gesture and slight bow. But, to my confusion, the kids then burst out laughing!
Having learned from that incident, when that Asian student greeted me with a bow I was careful not to bow back.
Now, next month, Andy Yao is coming to give a talk at Brown. Should I bow when I greet him? Would that be nice, or would it be misplaced? I have no idea.