I recommend twice yearly tunings for most people. Some of my more discerning customers prefer four times a year. There really is no "rule" about how often it should be done. But regular tuning will keep your piano from getting so badly off pitch that it will take several tunings to get it back in tune.
From time to time parts break or glue joints fail. Keys sometimes stick or dampers fail to work. All of these things are examples of routine repairs that Steve does on a regular basis.
As parts wear, adjustments need to be made to keep your action in good working order. A good regulation will make your action far more responsive.
Voicing is the adjustment of tone, rather than pitch. The two main goals in voicing are to achieve a musical tone to the owners' liking and to make that tone even throughout the whole piano. Some people prefer a very bright tone; others prefer something far more mellow. Still others may prefer something in between. All of these can be achieved through skilled voicing.
Sometimes a piano is so old and worn it is no longer serviceable. If it's a good enough instrument, it's worth restoring. Steve doesn't provide this service, but should you need it, he will help you decide first, if your piano is worth restoring, and second will help you find a qualified rebuilder.