I recently read an article by a prominent inspector that confused this issue as well. My understanding and opinion is that you want to initially seal the tile with a proper deep penetrating grout sealer during the install. There are many polymer type grouts out there that help keep the water out. None the less I would reseal the grout about every six months to a year. The idea that we should not seal the grout so the tile can breathe makes very little sense to me, it like closing the door after the cat runs out. If in doubt contact a tile expert as I have to clear up this issue.