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If the car does not need power then none is used. But if needs power it can get it. There is no downside to having it plugged in at least on 110 power.
While harder to do on a Model S, there have been a few Roadster owners who have bricked their batteries by not having it plugged in. But we are talking months here not days.
Let me add: best for battery for long sit is set slider to somewhere 50-70%. Not a huge deal, but purportedly averaging around 50% SOC preserves battery.