[aigel]

From first engineering principle, I cannot see how it "hurts" the piano. The force reduction on the string mounts and the support structure of the piano by sitting un-tuned for a few years cannot be significant. But Gogar has the explanation that makes the most sense. If it is sitting for several years, you will have to re-tune possibly more than once, because it will stretch out a little after that first tune.

Personally, I would compromise here and just have it tuned every 2 years. I would keep it in my home at that cost - like others mentioned, a piano ready to play may give you a nice moment with either your son playing it when coming for a visit or anyone else that has the ability when they are at your house.

G