Agreed with rcorfman about the mice issue - mostly a problem in well-used areas. But they will chew through anything to get to food they want, even a pack you are actually wearing!
Ravens also like to raid unguarded food, but won't usually steal food if you are actually there.
Think about a ratsack or Ursack and hang it for food storage if you want to be extra certain.
At Grand Canyon, the major campgrounds below the rim (Bright Angel, Indian Garden, Cottonwood) have food storage boxes, which simplifies things there.