I don't really know about there being any 'grades' of ceramic tile. There is thicker and thinner, basically.

As far as sealing it, check the box to see if the manufacturer recommends sealing it before grouting. If they do, seal it and then let the sealer completely dry before grouting. Then grout and the next day, seal the grout. Some manufacturers say to seal the grout and the tile afterwards, so follow what they advise.

The sealer will keep colored liquids from getting into the grout and also from staining the tile. Tile and grout are both porous.

Make sure you get bullnosed edge pieces so you don't have a sharp corner.