Aiya. Don't worry too much lar. If your are asking politely, most of the time, or rather all the time the US custom officer will be polite too, regardless of rejecting or agreeing to your request.
But since you are going to LA and SF only, i think where you land first in US is your destination, no connection. So you can have all the time to do whatever kind of check you want. But if you are connecting to an domestic flight immediately after landing, i think it is better don't ask for too much "trouble". Especially connecting at SF. It is quite big and very crowded. You will want to get to your connecting gate ASAP.
Anyway, at the checkpoint, you will see sign boards saying only ISO800 and above film will be affected. So asking for a hand check might be redundant for anything with lower ISO.
And if you have laptop and belt wth big metal clip, better remove it and place on the tray for scanning, and remember don't carry any metallic item with you when you going through the metal detector. If the detector sounds, then you are in for more "hand inspection". Hahaha.
I think you are transitting in Toyoko. So you better be very cute when you are checking in at gate 28G. Becos if you are not, you will be asked aside to do security check. And if you have a laptop, they will ask you to turn it on. So you better save some battery power for that. If it can't turn on, then you will be in a round of "chicken and duck talk". But don't worry, eventually you will still board the plane, but just how soon.