Some pianists prefer “bright” voicing while others prefer a mellower sound. Regardless of its original voicing, every piano will acquire a somewhat brighter tone with time, because the hammer felts will be compacted as they are thrown repeatedly against the strings.
Therefore, depending on your taste and the amount of use your piano gets, it ought to be voiced when your ear tells you the time has come. Experience has taught us that a piano which receives normal use in the home will need to be voiced every 2 or 3 years.
In the voicing process, the resiliency of tile hammer felts is adjusted with special tools and then all notes are balanced so that the tone is uniform throughout the keyboard.
This is obviously a job which demands special skills and should be attempted only by a professional piano technician.