The card was not issued with dirt on it...
I have on many occasions used a damp tissue to remove dirt from the surface of a card... the same thing they do to old paintings, they remove the grime to bring out the original color. Its not altering the card or painting, its cleaning it and leaves no residue.
But I wouldnt soak the whole card to remove a stain... its a bit risky, and some stains will actually spread more.
I would only soak a card completely if it were glued into a scrapbook..... to remove the back paper and as much of the glue as possible.
To me the risk of a little water damage is much nicer than a huge paper pull from "dry pulling" a card from a scrapbook. If a grader cant tell there was water used, its all good by me.
Even if they detect a water stain, they only downgrade, they dont reject.
Heck many Cuban cards have been rained on and have mildew stains from the humidity too. Water is natural, but can damage too.
Altering = trimming a piece off, recoloring a bad spot or print defect, rebuilding corners, filling in pinholes, or gluing rips or a paper pulls back down, rebacking (ie N172s), or chemical bleaching out stains (ie E145's), etc.