Hmm...
A screen protector would pick up dust, too...
The bare screen surface is glass, and not particularly easy to scratch, if that's your concern.
If you leave your iMac unattended for days at a time, then it wouldn't hurt to cover the iMac with a simple sleeve of some kind. It would be simple to put together something that would slip over the top of the screen to keep those evil dust-bunnies at bay.
That's not something that I have ever used on an iMac in more than 20 years of Macs, although I have had a couple of service customers that used something that they found, like an old "toaster cozy"
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