Different hobbies have different standards to be sure. And for many hobbies those standards have changed over time.
In coins, it's not so much cleaning, it's cleaning done poorly. Most silver coins you see that are bright and shiny have been cleaned, usually by dipping briefly in a very gentle jewelry cleaner then rinsing with clear water. It's not talked about much because its so common.
Removing a bunch of brown tarnish is pretty much ok, it is sort of an alteration, but the tarnish will eventually do a bit of damage.
There's a sometimes huge premium for coins with tarnish that leaves a very pretty surface, sometimes like a rainbow. Removing that drastically reduces the value.
"cleaning" with Ajax, Brillo pads, pencil erasers.... all totally no good.
Stamps have a similar view to cards, alterations of any kind are bad. Some stuff is accepted though, like if it's used and has been hinged into 5 albums with the hinge remnants left on each time it's ok to soak off the old hinges. Removing a hinge remnant form an unused stamp is a bit trickier.
Cars - It used to be almost required to "restore" even really nice originals. Now there's a big bonus for "original", probably because so few truly original ones are left. It was also pretty hotly debated when the money for some got to be enough that a car could be "restored" from nearly nothing. I've seen an occasional before picture of a what's mostly a truckload of rust that became a car.
Usually allowable restoration is more in the line of conservation, removing at stabilizing things that would in time destroy the item. It's common to have very old posters backed with linen. As far as I know the process is reversible, and keeps the poster from getting torn.
And as mentioned above, disclosure is usually very important.
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