I've done this quite a few times. It's always a nice treat to be on a flight empty enough to do it. Often times the flight attendants will invite passengers to "spread out" once the door is closed... but as others have suggested, it can be helpful to make a strategic move sooner than that (i.e. take the middle seat so neither the window nor aisle look attractive to others). Worst case scenario, you play dumb and move if someone else joins you in the row.

Once, I was even lucky enough to make camp on all four middle seats on an L1011... it was very nice!