Certainly any piano should be tuned at least once a year. If the piano is very out of tune, it may need to be gone over more than one time to make it sound really good. With the changes and temperature and humidity, the effects on your instrument will cause it to go out of tune.
By having your piano tuned on a regular basses, your piano will sound good for most occasions.If you have a piano that gets lots of play, or has other problems such as: loose tuning pins, you may need to tune more often.