If the table has a hinged dustcover, it should be down. Look, it's your table...do what you want! I am sure that the reason that manufacturers included a hinged dustcover was so that it could be closed while in use.
Or so that it could be opened to actually place an album on the platter and then play it open or closed and see which they prefer.
Very few absolute black and white ( open and shut) cases where audio is concerned.
But the only true test is to try both ways in your own system as it could give an entirely different result in another persons system depending on a whole slew of factors outside the table and dustcover.