I have been stopped at SFO customs the last 2 times through. August was an agricultural inspection and September was a "Random" selection by the useless guy that stands at the entrance to customs rechecking your passport.
I have gone thru LAX 5 times been inspected 1.
SFO 5 times been inspected 2.
SJC 3 times been inspected 1.
BOS 1 time with no inspection.
ORD 1 time with no inpection.
SEA once with no inspection.