Your piano sounds different than when you purchased it.
You don't like the sound even after it has been tuned.
The tone varies radically from note to note.
You cannot achieve a range of tones (mellow to bright) at different volumes.
The piano has lost its ability to play softly.
Before deciding if a new piano needs voicing, make sure it is well-tuned and well-regulated. Then, play a wide variety of music on it. Most voicing procedures are long-lasting, so give yourself some time to explore the sound of a new instrument before deciding to change it.
How often voicing is needed depends upon the piano's usage and its intended audience. Pianos in concert halls and recording studios often receive minor refinement of the voicing before each performance. A home piano may need some initial voicing to customize it to the owner's taste, then once every one to five years to maintain its tone.
Your piano and your musical needs are unique -- your own schedule for periodic voicing is a matter for you and your technician to decide. To find out how voicing might improve the tone of your piano, ask for a demonstration on one or two notes.