In general, extreme swings from hot to cold or dry to wet are harmful to your piano. Dryness can cause the piano's pitch to go flat and the sound board to possibly crack; moisture makes the piano sound sharp and may cause the keys to stick. Repeated swings in relative humidity can create distortion of sound and unnecessary creaking. Talk with your piano technician about the necessity of a humidifier for your location. This does not have to be expensive and will save you money in the long run.