Sealing polished porcelain floor tiles.

Hi all. I read somewhere on this forum that unless porcelain tiles are actually glazed, they will need sealing even if they are highly polished. I've just come to the end of laying highly polished porcelain floor tiles in my hallway (9sq m). The tiles are an off white and I've noticed a ring mark where the wet underside of a measuring jug I've been using must have sat overnight. Before I started, I did the overnight sauce test. There was a very, very faint mark but I decided not to worry about sealing the tiles. Also, I called the fella at the warehouse and he was like, "No, na, no, you don't seal them ones, you'll ruin them and they'll go all sticky". Seems now, though, that I will have to seal them after all. Can anyone tell me what type of product I should use and any specific brands. I want the highly polished look of the tiles to remain that way. Thanks in advance.

PS Any ideas on how to get rid of the mark? I wonder if bleach will work?

If anyone is interested, the bleach got rid of the stain and left no trace. I would be grateful if someone could offer the advice I requested. Thanks.