mmayer's advice is spot on, but I'll disagree on one point. I wouldn't worry about it at all. There are a bazillion later flights in the day to SFO. Also, not all of them go into terminal 2 by any means. The low C gates are all express, as are the high B gates. Take a look a couple weeks in advance and you'll get a good idea of the area it comes into. Most flights tend to go into the same general area (pretty sure each express carrier has different gate ranges).
The ExPlus flights go in to B/C and the non ExPlus ones go into F. ORD-SFO flights can be VERY full so I wouldn't play roulette with a cruddy middle seat much later in the day if I could avoid it.