If the tiles are glazed then you should have no staining problems. Some of the cheaper porcelain tiles about today are more porous than they used to be so if they're if unglazed then you should consider sealing.
If in doubt, do a test. Leave some soy sauce (or something similar that you know would stain something porous) on the face of a tile over night. Then in the morning, see if it wipes off without leaving a stain. No stain, don't seal. Stained, seal the tiles.
I would have thought though that if you've been told by the supplier that these tiles do stain, then they're probably telling you that because they've had a customer complain about them staining.