Why Not?
Did you check the porosity of the surface? How old is the slate?any stains previously?
Grout?-wide joints? condition of grout? never hurts to seal the grout if you can get it clean. Sell them a color sealing job.
If the slate is roofing quality material or just non-porous then you know the answer.However maybe the customer wants to pay you so they can have piece of mind. At any rate do the testing so you can offer them the options in a credible way.
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