Cleaning is a topic in other hobbies, but doesn't get as much discussion. It has at times in the past.
In coins, the old rule was "never clean a coin" And yet, silver tarnishes easily, and there are thousands of pretty old coins that are nice and shiny.
As the dealer I go to and in college worked for explained it, if the cleaning is done in a way that doesn't do any damage, it's considered ok. So many people think that their work "cleaning that old penny up to sell it" is fine despite being done with a pencil eraser...
There is also currently a decent bonus for coins that have really nice looking tarnish(toning), some of the envelopes that used to be used to put coins in give them a rainbow like sheen.
Stamps are closer to cards, but have a few technical issues that most cards don't have. Some German stamps had acidic gum that has been known as something that will eventually destroy the paper for a very long time. The Michel catalog (The primary German catalog for German stamps) specifically states that the gum should be removed.
It's also common to remove stamps from pieces of envelope by soaking (Just make sure the cancel isn't one that adds a lot of value. )
And some series are typically dipped in a solvent to see if it has a watermark and if so which one. Or to look for damage like thins and creases (Some creases naturally get pressed from the stamp being in an album for 50+ years) doing that also typically washes off some surface dirt.
So it's sort of accepted.
A cleaning to remove stuff that really doesn't belong on the card and that will easily come off - like a wax stain on the front, or a bit of elastic band that's gotten stuck or general airborne dirt it's picked up over time should be ok.
Overcleaning probably shouldn't be ok.
I'm really on the fence about things that will prevent a decaying item from further decay. An example and a bit of an explanation
https://pastispresent.org/2014/libra...testant-tutor/
https://ritaudina.com/en/answers-on-...-deacidifying/