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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VPI Classic 2  gives me  few options.  It is either on, in place, or completely off.  It is not hinged or fastened  but sits in place held by its weight. I almost always take it off.  Sometimes leave it on when rambunctious kids or dogs are visiting. 

BTW, as dust covers go, it looks OK, but has some curiously placed open spaces, like screen doors on a U-boat.  Better than nothing, but could be a tighter fit. 
I ordered a custom dust cover for my table....heard a reduction of fidelity....too big for a hinge.   Off or on.....I sold it a month later.