A pre-grout sealer is not always necessary for slate. You typically use pre-sealers on highly porous tile such as tumbled marble or brick pavers. However, you certainly can use a penetrating sealer or pre-sealer designed for slate and other stone. One scenario where I would recommend pre-sealing slate is if the grouting will be taking place where there is heat and direct sunlight. Pre-sealing will help keep the grout from sticking to the surface of the slate too quickly under those conditions.