How often should my piano be serviced?
That depends. It is recommended that pianos be tuned twice a year approximately the same times each year (see Why does a piano go out of tune?). Some pianos are very stable and can get by with one tuning per year.
New or rebuilt pianos need tuning more often. The new strings and wooden parts in the piano need time to settle and stretch. A new or rebuilt piano needs 5 or 6 tunings before it becomes stable and should be tuned at least twice a year (or more depending on your requirements) for its first 2 or 3 years. Some manufacturers require that their pianos be tuned twice yearly to keep the warranty in effect.
Piano stability is something we should aim for. It is best that the piano is tuned at the same time(s) each year because the humidity levels will be similar. A piano's pitch fluctuates with differing humidity levels. With the humidity level (pitch level) similar each time I tune, I need to move the strings and tuning pins less. If the movement of the strings and pins is kept at a minimum they tend to "stay put", resulting in a piano that consistently remains in better "tune" throughout the year.
It won’t hurt a piano if it isn't tuned regularly but you won’t be terribly inspired to sit down and play music on a piano that is out of tune. A properly tuned and maintained instrument can make playing or listening to the piano much more pleasant.