Cleaning the outside of an iMac with an electrostatic cloth
Personally I would never use anything that can build up an electrostatic discharge around computers.

While the power supply may be 240~120V AC nonetheless the internal component are running a 12V DC or less and at the point an electrostatic discharge occurs that causes a spark and you feel the discharge it's at ~1000V DC and more then enough to permanently damage the electronic components of a computer if discharged through. This is why we that work in the industry use electrostatic mats and wrist straps when working on computers.

Through my career I've witnessed many times a component getting zapped because proper electrostatic discharge precautions were not taken by some colleagues and many users. I too have zapped a thumb drive via electrostatic discharge accidentally touching the USB connector having just walked down a carpeted hallway in the winter time while in northern climates because I forgot to ground myself first before plugging in the drive. I felt and saw the discharge as I plugged in the drive. It was working two minutes earlier in the computer I properly removed it from.

So, even just wiping the outside can have potential if it can discharge while brushing over any of the ports.

I recommend using microfiber cloths that have been dampened and wrung near dry so as not to have enough moisture to damage the computer too. I've been using microfiber cloths for more years then I can remember and have never damaged anything I've used them on.

Additionally, especially if in climates conducive to electrostatic discharge, take proper precautions and at a minimum, wear an electrostatic discharge strap making sure you've grounded out any built up charge before touching the computer and maintain that ground while touching the computer.

BTW Sorry about the original answer, it was meant for a different question. :)