I just saw the below sentence at Faust Harrison's Web site (piano care section) and I am a bit astonished.

"The fallboard should always be open to allow free circulation of air around the keys."

I always keep it closed.
Am I doing it wrong?
How do you keep it?

Regards,

Yes your doing wrong. When closed it "sweats" and can cause the keys to swell, making them stick.
Leave it open, a little dust will not be a big deal on the keys. They clean easy.

I'm not sure that closing the fallboard is going to give you such a hermetic seal that the keys would "sweat."

Also, if this were an ongoing problem, wouldn't manufacturers come up with a design cure by now?

It would be easy enough to vent the keys by making the fallboard fall less than 100% against the keys.

You know what? Close the cover.

I think the humidity issue would depend on where you live. Out here in normally quite dry west Texas I doubt humidity buildup would be much of an issue. However, it's raining tonight, so I'm going to open my fallboard now. Back in a bit . . .

With two cats and a four-year-old, I have to keep it closed most of the time. I'm stopping at three in a row. I promise.

From the 'How to Care for Your Piano' section of the Kemble web site:

Be sure to close the fall each time after playing.

I always understood it to be:

Ivory keys: Keep it open to keep them from yellowing.

Plastic keys: Keep it closed to keep them from yellowing.