I am trying to figure out what this says about the virtues (or otherwise) about grounding a PC while cleaning; I am not saying either is safe (for the PC), just wondering which is best.
Doesn't matter. What matters is ensuring you (the computer) are not connected to a power source. This is just another reason not to use the power supply's power cord. If you want to ground to "Earth" ground, that's fine. Just use a separate ground wire to "Earth" ground - preferably as separate "Earth" ground and not one [hopefully] provided through the wall outlet.
That said, I recommend just unplugging from that wall. That eliminates even the possibility of coming into contact with AC. Plus, you won't get your computer yanked out of your arms when lugging it outside to blast clean with your air compressor if you unplug it first!
Don't be mislead or fooled by this old wives tale nonsense! Volts kill too. Yes, it is the amperage that is the actual lethal component, but it is the higher voltage from the wall outlet that overcomes the resistance in the body and thus delivers (provides the "pressure" or "push" for) that lethal punch. You need both - but make no mistake, voltages as low a 42 - 50V can still kill.
But one should not make assumptions when it comes to electricity. While a 12V car battery will not "electrocute" a normal "healthy" person, it can certainly cause AFIB or even cardiac arrest or other complications for someone with a preexisting heart condition.
It must also be noted that time is a factor too.
Prolonged contact, even with low voltages, can kill. Fortunately, the natural reaction to coming into contact with electricity is for your arm to jerk back and slap yourself in the face.
Very true... but is this still true for electronics? A static spark will not harm a human but can destroy electronics.
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Some" electronics. Except for causing interference, (or igniting flammable gasses) static discharges didn't used to be a problem back in the day. The "phenomenon" of ESD damage to ESD sensitive electronics did not really exist, nor was it understood until the mid to late 1970s when the integrated circuit (IC) devices started to become wide spread.
When I was in tech school in 1971 - 1972, we studied static electricity, but nothing about "ESD" or "ESD" prevention. It was not until high-density "solid-state" IC digital devices started to become commonplace in our military communications systems
AND components that failed under "mysterious" conditions were being analyzed under high-powered microscopes, did damage from ESD become recognized and understood. I was sent back to school to learn about it, and how to prevent it.