A piano owner in Sheffield just emailed me, asking how long he should wait before getting his second-hand piano tuned after delivery. I replied with the following (edited slightly):

Piano dealers will often tell you to wait a while (usually a few weeks, but in this persons case 3 months) before getting your piano tuned to allow to it “settle into its new home”. Their view is that the piano will not hold its tuning very well until it has gotten used to the changes in humidity, which affect the soundboard and the inner workings of the piano. While this is true, changing temperature and humidity will subtly affect the tuning of the piano all year round, and in my experience, the extent to which these changes affect the stability of the tuning are often exaggerated. Speaking from experience, the extent to which the strings are tightened has a more drastic affect on how long the piano stays in tune than does the room’s atmospheric conditions. Leaving the piano months without a tuning will ensure that, when the piano tuner is finally booked, the whole piano is even more flat that it would have been. Then the piano tuner will have to stretch the strings even more than they would have done, putting more pressure on the soundboard resulting in a less stable tuning. For this very reason, piano tuners recommend that a piano is tuned at least once every six months. This way you will have a piano kept close to concert pitch and will stay in tune for longer after each tuning.

– Richard, Piano Tuner Sheffield.