I had a bit of a strange experience when walking to work last week. I was passing some guy on my left when he bowed at a person who must have been right behind me and said "あ、どうも、おはようございます" ("Oh! Good morning!"). I was looking forward and, out of the corner of my eye, it kind of looked like he was bowing at me. Before I had time to even consciously register what I was doing, I was already leaning in to bow back. It was only then that I turned my eyes towards him to see him looking towards someone behind me, and thought how odd my reflexive action was.
I think that was the body language equivalent of the automatic reply of "Hi!" when someone says "Hi!" to you. You often don't even think about it before it comes out.
In learning a foreign language, it's exactly that kind of thing that you should be striving for—in both spoken and body language (which of course is an integral part of learning a language); common social communications should ultimately be coming out so naturally that it's almost Pavlovian. And it's no less true if it means that you end up half bowing to random people on the street.