the rice is cooked already and there's vinegar in it. choose those cooked ones like unagi or crab meat and egg.
i dont't think cold is a factor for bacteria.. i still eat cold sandwiches. subway veges are chilled too.
(i ate sashimi when i was pregnant. it's a risk to take, and i dont't recommend. i ate a few bites. too tempting. lol! lucky nothing happen.)