According to my tech's advice, I close the top at night. A lot of the time I leave the case closed, and have a freestanding music rack and a very nice piano lamp on top of it. In that case, I only have to deploy the key cover. This easy-care approach is greatly appreciated in dusty California, but when I do have to dust, I use an ostrich feather duster. You might have to order one by mail, but stay away from those turkey-feather affairs; they will scratch a finished surface, dust or no dust.

I use Cory's Piano Polish; it does help protect the finish from dust scratches. But over time, there are bound to be some. So, what's really the important thing about your piano?