It's an unwritten rule that you don't move seats before the show has started unless an usher specifically tells you that you can. It can create havoc if there are latecomers during a performance. Once intermission comes, whatever is unsold/empty is generally considered fair game.
As others have said--it all depends on the theatre, the ushers, the performance. When I saw LA BETE, I was seated in the obstructed view box where they placed people who bought general rush, and the usher moved me and my companion to front row center mezzanine seats as the curtain was about to go up. There were maybe thirty people in the mezzanine all told and the boxes were completely cleared before the performance even started. A similar thing happened at the performance of THE NORMAL HEART that I saw, when the ushers tried to dress the house a bit by encouraging rear mezzanine patrons to move down to the mid-mezz. Given that the Golden is such a small theatre, my companion and I decided to stay in our second-to-last-row seats.
Come to think, I don't think I've ever been told to return to my seat the few times I've moved during an intermission.
Sean is correct, though--you should never try to move seats at The Met. Those ushers watch the audience like hawks, and even if an entire section of rows is empty, they'll instruct you to go back to your ticketed seat if they see you move.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body
