With busy holidays (performances anyone???) and the flu season always lurking around the corner, it is not a bad idea health wise to wash your hands before and after any piano practice when you share the instrument with others. Hand washing prevents a lot of colds being spread and you don't want to catch a cold in a busy season.....
Also, it helps keeping the keys clean (I have a toddler banging on the piano every now and then. Yuck, what a mess on those keys sometimes!).
Angie