Turntable lid up or off?


My Pro-Ject Classic turntable came with a dust cover.  I usually spin vinyl with the cover completely removed and never closed.  Does it matter if the cover is just up or fully removed?  The ultimate test is just listening, but I cannot detect a difference between either mode (up or completely off).
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Then they say things like "it will sound closed in" Brilliant deductive reasoning.
Or, they’ve listened and came to that conclusion. OP, try it yourself. Turn it up. Play a track you know well with the cover down, off, and down again. 
I keep it down when listening to my SL-1200 MK2. Most of my other tables don't have dust covers. Up seems like the worst option. What sounds best to you?