Strip Cards, To Clean or Not to Clean?

[Flyingace]

As a collector and part time dealer of baseball memorabilia for many years (full disclosure), I have questions on cleaning W strip cards.
A large collection was brought to me, mostly in very poor filthy condition. The cards were collected by various members of a family, and had been in a dirty basement for decades, many wrapped in rubber bands ...
The family wishes to sell the collection, however, the cards need curating, i.e., surface soot and other crud, and dirt must be removed to make many of the cards have value.
Members of the family are concerned that cleaning will devalue the cards. Soaking is out of the question, however, there are some wonderful cards here, which will benefit from being cleaned. Of course cards that do not have soot and crud on them are worth more than dirty cards. Not talking about using chemicals here, just light cotton or q-tips, maybe water.
As I am not being paid for this, I have no skin in the game other than my time. Your advice, please.