In large shallow dish, spread rice; sprinkle with seasoned rice vinegar and sesame seeds. Using wooden spoon, toss gently to combine (do not stir or rice will clump). Keep rice covered with a damp cloth if not using immediately.
Using wet fingers, place rounded teaspoon of rice in the palm of your hand. Make an indentation in the centre and place desired filling inside. Dip fingers into water and pack rice firmly around filling to make a small ball. Repeat with remaining rice. Sprinkle with toasted sesame seeds or dot of wasabi, if desired. Cover with plastic wrap and keep at room temperature until ready to serve.