In most cases a piano should be kept at ‘concert pitch’; that is, where A=440hz. This maintains the correct tension on the frame, and provides the optimum tone quality.
It is not the playing of a piano which has the most effect on its tuning, ( indeed, a well tuned piano should hardly be affected by even the heaviest playing. Achieving that stability is possibly the hardest part of the tuner’s art) – rather, it is changes in temperature and humidity which cause the wood of the soundboard to expand and contract, which throw a piano out of tune.
For this reason, a domestic piano should be tuned at least twice a year to counteract these changes. I normally recommend 3 times a year for most of my customers because a more consistently high quality of tuning can be achieved.