I can honestly say that I sit in FC 90% of the time when there are seats open. Board last and when you get on obviously introduce yourself to the lead FA, and just pretend like you were never assigned a seat and ask, is it alright if I just grab an open seat? It works great every time! Of course I am always loaded with the chocolate candies and goodies which helps. I have only had one instance where the gate agent assigned me first class, mostly the FA's.
