I always ask them if it's okay if I end and it always is. I've had one person ask me to look around and I let them, but my town was a complete, total and utter mess so I really felt awkward, but I couldn't say 'no' because this person was super-nice. Fortunately, it was someone who traded a lot so they were used to their town being cluttered with items all over the place.
I think it is uncomfortable at times because I am not a rude person by nature and I don't like kicking people I just want them to leave when they are done and have had time to finish talking or whatever, and I type to them on the board or pm while they are in my town, just faster for me. I have no problems with people shopping in my town for everything is right by the gate, and I kinda like looking at other peoples towns because there are so many creative people out there and I am not one of them.
It depends. If they're there for a trade, I like to make it business only. If they make an excited remark about my town and if I have the time then, I'll let them explore. If I don't have the time, I'll tell them they can come back to visit another time.
When trading with most people, they understand they may not always be the only person you are trading with at that moment. As long as you check that the trade has been fully completed and say goodbye, most people won't get offended if you end the session.