I can prebook a seat on my next flight. I would like to be able to be one of the first passengers to get out on arrival, just to prevent being last on the queue at immigrations. At some airports big planes are connected by at least two jetways, making it possible to leave from the backdoor. Is this also the case at SFO?
Short answer - For any airport with jetways / jetbridge / airbridges, sit as far forward as you can get!
Longer answer - a 777 has 4 doors on each side (5 for the 777-300), but in common with almost all planes and airports passengers only ever use the left hand side ones unless something has gone wrong. Those doors are 1L (at the front), 2L (about row 10-15, towards the end of business), 3L (just behind the wing), and 4L (at the back). (A 777-300 has 3L just above the wing, 4L a little behind, and 5L at the back)
If there's only one jetbridge available, it might use 1L, or it might use 2L, depending on the airport, never anything further back. If it's a dual jetbridge gate, then both 1L and 2L will be used. Typically when using 1L and 2L, boarding is split by class, and disembarking is done with one side of the plane using one door, the other side the other. For that reason, sitting at the front of the left hand side is normally slightly quicker!
The only time the rear door will be used is when steps are used. If steps are used, they're normally 1L or 2L along with 4L/5L. Steps are only rarely used with a widebody like a 777, and normally only at smaller airports on (eg "bucket and spade" airports on mostly tourist routes). Otherwise, unless there's a problem with the gate, at a big airport it'll be jetbridge at the front.