I actually talked to my T about this once. I live and go to school in the downtown area of a large city, and her office is a 2 minute walk from my room. I complained once because I'm insecure.. about an old T that I see around my home town quite often.. and how she never says hello and acts like she doesn't even know me at all. My current T says that that is the norm for some Ts because they don't want to create an awkward situation for their clients. She also said that she usually asks people if it's okay to speak with them if they bump into each other in public, and based on their answer, she either says hello or doesn't. That said, it's really all up to you, and what you're comfortable with. Hope this helps
Ella