Does Light Washing Dirt Off Devalue Coins?

The usage of distilled water for car batteries or any rechargable Lead-Acid battery is actually a great example of why you should only use it on coins. As a battery charges it emits gasses and usually some of it is water vapor. It is replaced with Distilled water for a reason. Using any other type of water will slowly create residues on the bottom of the battery that will shorten out the terminals.
That samd STUFF in tab, well, spring, etc waters is what also is not good for ocins. If left to evaporate on a coin, that reesidue left may well ruin a coin.
Distilled water is labled as such on the container it is sold in at almost any store.
Blow drying with a hair dryer on warm will help to evaporate moisture faster and also blow off some of the moisture too.
Always note that few soaps ever really tell you exactly what they are made of since allowing that info out would possibly help their competiion make the same.
Even tooth brushes are not that quality controlled to make sure every strnd of a soft one is really soft. It would only take one strand of course to damage a coin.
Anything past an Acetone and distilled water dipping could ruin a coin.
Coins are not exactly made to withstand the same treatment as a railroad spike.

if the coin is so dirty and you cant read the date the coin does not have much of a value over the metal content.

I see a big difference in " cleaning a coin " and removing caked on crud .

the most non abrasive way would be the soaking technique. Just enough to get a date , or date and mint mark readable then decide to go from there.

Yes I've read mentions about soaking in olive oil ;) I've also seen mentions of using a softened toothpick to gently try get caked on dirt off from around the edges of devices, but I do worry about leaving scratches.

A lot of the coins I get have a lot of dings and nicks because they've been heavily circulated for a whole lot of years so sometimes I risk that if they're low grade looking and I really want to see some more detail.