Hi Kim:
I hope I am understanding your question correctly. I use open seating most of the time and the children choose where they want to work. But I introduced assigned tables this year as well. I showed them the poster of the assigned tables, and who went where. Each table also has a table captain, responsible for making sure things get put away well, passing out papers etc. They are assigned by day of the week. For certain tasks, lessons, etc,I simply tell my students that for this activity we will sit at our assigned seats...or more likely, they will ask me! There is no problem when this seating is necessary since they have freedom of choice most of the time. If Math needs assigned seats, then that is simply part of your math procedure. Hope this helps.