I keep mine covered for the most part. Sometimes I am just tired and forget. If you've ever looked inside a desktop computer after a long time you'd wonder how in the world all that dust got in there. Server rooms are another place I've been where I've seen equipment failure due to a dusty environment. I picture a mini grain mill in my machine that if there's too much dust it will burst into flames. Ok so that's a bit of a drastic scenario but still the same concept. Kind of like a clean car runs smoother. I figure why risk it? Only takes a second to cover and uncover. Of course me being the obsessive that I am I made a cover, thread catcher, notepad cover, and pincushion all to match.