In order to ensure proper A-440 tone all acoustic pianos need yearly, bi yearly or quarterly maintenance.  The number of piano tuning visits needed per year depend on a couple of different variables.  The Quality of said piano, Temperature and Relative Humidity of the environment in which the piano resides, The regular piano players ear,  and audiences expectations.  

A piano tuning is a lot like an oil change for your car.  It allows for preventive maintenance and is a great way to protect your investment, but it is not the end all be all of proper maintenance.  From time time other systems of the piano will need to be addressed to guarantee a great playing experience.  You might need to Regulate the action or dampers but with a little regular maintenance you piano will play great for years to come.