When talking about "Tu", that's almost true. (But some regions in the country still use it, mostly in the south).
Now, when talking about "Teu/Tua", "Teus/Tuas", they are indeed used.
But there's no rule to decide between "teu" and "seu".
However, "Teu" assures you're talking about the second person, not about one of the other objects in the sentence (if there are other objects).