The first person on the floor usually has the right to set the dance they want to do. However as a courtesy so everyone has the opportunity to do what they want, I suggest that if you think lots of folks know the dance you place yourself in the center of the floor. If however, you feel very few know the dance you want to do, line up on the edge of the line dance area. This gives us better use of the floor by not having people all around you trying to do a different dance than the one you're doing. The initial logical feeling is to face the music.
Dancers joining lines have a tendancey to join an already existing line rather than form a new one.
Lines facing the longer edge can stretch out farther. It's a fairly accepted practice that when a colision occurs, appologize, EVEN if it wasn't your fault.