It is a fact that a piano will go out of tune whether it is played or not. There has not been a piano ever made that does not require tuning.
The main reason that pianos go out of tune is due to changes in humidity from season to season, affecting all pianos, new and old, played and unplayed.
Pianos go flat in the winter months when dry heat expelled from your radiator draws moisture out of the piano's soundboard. In the spring, when you turn the heat off, the air is usually more moist. The soundboard absorbs this moisture, expands and causes the piano to go sharp by the summer. These seasonal changes in tuning are often most obvious in the mid-range of the piano.
Fluctuations in room temperature surrounding the piano cause less of a change in tuning than humidity changes do. But, direct sunlight or heat from stage lights can cause rapid changes in tuning.
When you move, it is not so much the transportation of the piano that throws the tuning out as much as the piano acclimatising to its new room environment. Wait about 2 weeks after you move before you get a tuning.
If both humidity and temperature are controlled in the room where the piano is situated, these swings in tuning virtually disappear and your tuning is much more stable. So is the overall consistency of the touch response you'll get from the keyboard.
New strings can cause the pitch to go flat. New piano wire is quite elastic and starts to stretch as soon as it is pulled up to pitch. This is why new pianos or pianos that have been restrung need to be tuned more frequently in the first year. Each time the wire is pulled up, the amount of stretching decreases and the tuning becomes more stable.
Slipping tuning pins can cause a piano to go flat. Older pianos that have been exposed to regular seasonal humidity changes over the years can have loose tuning pins and as a result, have poor tuning stability.
The louder and more often you play a piano, the faster it goes out of tune by a small amount. The force of a hammer repeatedly hitting a string can affect the equalization of tension along the string's length, and cause its pitch to be slightly altered.
To put the matter of tuning in perspective, remember that a concert piano is tuned before every performance, and a piano in a professional recording studio, where it is in constant use, is tuned 3 or 4 times every week as a matter of course.