More often than not, if a seat clears the gate agent will come reseat you before the boarding door closes. I imagine this doesn't happen 100% of the time so normally if I know I am next in the list I'll ask the gate agent if I should go ahead and board (they will usually say go ahead and board and they'll come get you). If the flight might be delayed sometimes I'll just wait to board since that is probably the highest risk situation where they might leave a seat unoccupied in the event that there are no shows to get the flight out on time.