Today I learnt a very important lesson concerning that question.Princess Trollestia said:Do you? I know I usually do, except when I take a #2. Here's a pic for those who don't understand:
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I took the freedom of editing the provided picture in order to make my point clear.
When you leave the lid upon during your physical / mental absence from the bathroom, it opens the energy flow to the water in the toilet what enables ghosts and other nasty stuff to creep up from down there.

By closing it, you cut the stream of energy, and by this you prevent uninvited entities from entering your private sphere, since they stay trapped within the toilet.

Conclusion: Close that lid.
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Serious answer: I usually leave it open.