Porcelain polished 60x30 wall tile sealing advice pls.

Hi all, I've read most of the historic posts on this and I'm afraid I'm non the wiser opinions differ as to whether they need sealind at all. I've just bought a LOT of grey homebase 60 by 30 polished porcelain tiles which I'm going to "brick" pattern on the walls, floor to ceiling, in my shower room. The floor is also being done in the 60 by 60 white ones.
I've decided to use aquaboard on 1 and a half walls and moisture resistant plasterboard on the ceiling and remaining walls, i wasn't intending to tank it as its just an electric shower with a huge tray and im using 2 mm spacers... tanking for me is for a wet room and aquapanel say you don't need to tank? Anyway that's probly gonna get some comments, but my real question is this.
Sealing the tiles. The box says "don't forget to seal your tiles" if i accept that they should be sealed (that's questionable if you ask some) then should I do it before hanging, on all edges, after or before grouting and what stuff should i be using for best results on an already poished tile. If you never seal them, what happens?
I'm going for gunmetal grout btw. (cement based powdered adhesive obviously)
Any opinions apprciated...