Stone is pourous and needs to be sealed on the top surface before setting. This makes any grout cleanup easier. You should select a penetrating acrylic (waterbased) sealer specifically for slate tile and apply at least 2-coats. After setting the tile and floating the grout, the stone can be cleaned relatively easily. After the grout is cured it too needs to be sealed to repel dirt. When I set stone or saltillo tile, I will usually use a compatible polish which reinforces the sealer, but provides a strippable surface for cleaning.