It's a big deal and shouldn't be done ever unless it's plain water being used to remove excess foreign material, or from a scrapbook.
There are cards in slabs that were cleaned with chemicals now showing signs years later of regression (stains coming back, creases popping back up etc. And no pressing out creases never truly removes them) and in my opinion pouring chemicals on paper probably accelerates natural deterioration of the paper. Watch Orlandos video of his card he bought that had no stain originally but now shows the nasty stain that was chemically removed years prior.
Not to mention the clear loss in clarity that occurs. Sure the cards look brighter, but dramatic detail is lost, again showcased perfectly in Orlandos video. I can see it the most around the subjects face it is almost akin to taking a card that is perfectly focused to slightly out of focus.
https://youtu.be/KYCkxXUb9u8