I like to face the main entrance and have a minimum of open space behind me, like a corner in the back. I also sit on the outside of the booth or table. (so it is easier to get out, or protect the person sitting to the inside) Away from outside windows when possible.
Basically maximize visibility of the area, and potential entrances and exits, limit blind spots, and limit your visibility to others like passers by outside.
I don't make requests for seating, I usually make the best of the location I am given. Though I would make a request if I felt uneasy about the situation/area... but I usually would just avoid such situations in the first place.
This is habit from military training... They taught this for when we are out and about when deployed abroad, and it just sort of stuck.
They also taught basic situational awareness as well... turned out to be good training for CCW in the civilian world if nothing else.