The main problem with having a selection for each board is this. You can't stuff it into cookies, because once you have a certain number of forums you'll reach the cookie limitation. I had this problem before when we used to store the last visit time for each board in cookies. The other problem would be storing it in the db. This would require a separate table and then you'd need to do another query when going into that board to grab their preference. Adding more queries is something I try to avoid whenever possible.
