However frequently your piano is used, I should be tuned at least once a year in order to keep the balance of the pressure applied to the soundboard by all of the strings. Not tuning a piano annually can lead to false economies.
A piano which has infrequent tunings will need a more in-depth and longer tuning, therefore becoming more costly in the long run.
The mechanical parts of your instrument are highly solicited by the repetitive hitting of the keyboard. The felts on the hammers pack down and flatten, taking away from the precision in the sound; the rotation axels (pivots) of these hammers also finish up with some play in them and have an impact on the tone. The numerous regulation points along the whole of the mechanical parts / keyboard should be regularly check and rectified in order to maintain the optimal potential of the instrument. When the tuner comes by, he may advise you on any potential work that way need doing (filing of hammers, pivots, mortaises de clavier)