No matter how much freeze-dried food or grain you may have stored away in your survival stores, if a nuclear war comes to pass, sooner or later your food will run out. Food in sealed containers would be safe to eat if you were careful to wipe off any fallout dust on the container before opening it. Radiation doesn't make food dangerous and only slightly alters it so that it loses little of its food value. But chances are good that any store will be stripped during a pre-war panic. Even if it were full at the time of the attack, time is against you.
Foods have a finite life during which their nutritional content remains high.
Once this time is exceeded, the nutritional value of the food gradually drops off. Food will remain eatable for some time but it will not necessarily supply all your nutritional needs. Nutritional shelf lives of stored foods are short. So even IF you have a grocery store to use for supplies, the nutritional value of the food will be nearly nill after a year. But if you're in an area where the plants are producing enough food to support you, chances are good that there'll be a large human population as well.