Personallly speaking I prefer to seal the tiles prior to laying that way, as I know I'm a mucky
tiler, I'm not likely to stain the tiles in anyway.
Don't use a wet cloth to wipe off the dust as this could again stain the tile, use a lint free cloth or closed cell sponge (one with tiny holes not great big fat ones)
Seal north to south, leave to dry then seal east to west, creating a lattice effect - that saves creating direction marks
DO NOT seal the sides or underneath - this will stop the adhesive and grout from adhering to the tile
Seal as many times as the tile needs to be done until it stops readily soaking the sealer up
Lay, gently wiping off any adhesive from the face,then seal n-s e-w again before grouting, and then adding a final seal
Longer way round but pretty much failsafe
Remember also that the sealer will wear off over time, so will need stripping back and resealing again....