There are many ways to get free food on campus and outside the campus. The best way to get free food on campus is to go to Info Sessions held by companies to court engineers, or through the many little events held through other the year by many campus student organizations.
In most likelihood the people who present these things are not going to remember you. So it is definitely fine to go to these events just for the food. In all actuality you might learn something cool from these places and many a person has found an interest in a company after going to the info session for the food.
These events are usually held in Cory or Soda Hall and held through out the week.
The "quarter meal"
The "quarter meal" is served Monday through Friday at 2362 Bancroft Way, 1st floor (enter through parking lot) by the Berkeley Food and Housing Project. The meal is free; it is named the "quarter meal" because at one time each diner was required to bring a quarter.
...Also, check out free food on Oakland Wiki!