No. 

Sealing buys you substantially more time to clean up a spill before a stain occurs.  If you spill oil on a sealed floor and leave it over the weekend it may still slowly penetrate the seal.  Oil on an unsealed surface will stain. 

Stains may also be caused by situations behind or beneath the tile.  If you have nail that is trapped in the mortar that starts to rust it will stain the stone from the backside.

Most stains are caused from top-side spills.  Sealing addressing these type situations very well.