Piano tuning is primarily for the player and the listener. If your piano is being used, I recommend tuning it at least twice a year. This will facilitate proper musical development of the ear and the most enjoyment of the instrument. (I have frequently had customers remark that their piano is fun to play again following a tuning.) Students who are practicing on an out of tune instrument are more likely to become frustrated and to quit. Regular tuning also allows your tuner to catch and correct any problems with the instrument early.
As discussed on the ‘What makes a piano go out of tune?’ page, the tuning of your piano will change dramatically twice a year, corresponding with the beginning and end of the heating season. Tuning your piano twice a year will keep it reasonably well tuned throughout the changing seasons. (Music professionals, serious players, and institutions will usually have their pianos tuned even more frequently.)
On the other hand, if your piano is not being used, I don’t believe it is necessary to have your piano tuned as regularly. A piano will tend to drop in pitch over time. If you wait too long, there may be an additional expense for extra tuning when you do decide to have the piano serviced, but lack of tuning will not damage the instrument as some tuners say. I recommend tuning a piano that is not being used perhaps every 2-3 years, or right before you know that it is going to be used (e.g. when your child is coming home from college for a holiday break).
Here are some other opinions on this topic from around the web: