Dust cover up or down?

If the manufacturer provides a hinged dust cover, they must be saying down when playing. A non-hinged one or one like my Rega P8 is saying off when playing but on when not to keep dust off. Some like the big ones over everything not hinged to be on while playing that don't touch the plinth or base which might be better at eliminating air borne vibrations and accumulating dust while playing, but what a pain moving them on and off when flipping a record - double worse for 45s. Unfortunately in our hobby there is no perfect solution....to anything. Just keep trying to get as close as possible.

My experience is, it all depends.

With my Belt drive Music Hall MMF-5 I had to play a disc with the dust cover off.

With my direct Drive Technics SL 1210 GR it does not matter which way, closed, off, half way open, it's all the same. No effect on the sound.

soko, I don't agree with your interpretation.  There was a time in the history of audio per se as a hobby, let's say since the mid-60s,  when the dust cover was an expected part of a turntable, and many manufacturers provided a hinged attachment for convenience.  I never took that as an indication that the manufacturer was recommending anything.  It was just a feature that might seduce a buyer.  In many cases, where there is a hinged DC, the hinges are designed such that the cover can be totally disengaged from them to allow for their complete removal.  I wouldn't over-interpret that, either.

Off

Goes without saying. Off!