As a rule now, we try and advise sealing all polished FB Porcelain. Even though some manufacturers say they're pre-sealed, you don't know what with or whether it was only temporary for transit purposes and will only see you through to grouting. As an example, after a few complaints, we found out that one of our ex suppliers has been sealing their tiles........ with duck fat!!! Yeah, mop that with a good de-greasing floor cleaner and see what happens!
You will do absolutely no harm in applying an additional coat of something like the LTP MPG. Better to have ones posterior covered I say.