helpo1: Theoretically, no. You don't have to wait until the 5th card. You can know if you won even right after the flop. If you got the royal flush. :P
I'm trying to think of another scenario where you could be certain you'd won before the river card was laid down.
What if you have two aces on hand with one in the flop? If, after the turn card is laid down, the three other public cards are all below 10 with one each of the three other suits than the ace on the table? Wouldn't that be an absolutely certain win?
Edit:
D'oh! No, it wouldn't. If the river is the same value as one of the other cards, someone else could have four of a kind. Never mind.
Edit2:
Okay, new scenario. The turn card is on the table and you have a low-value straight flush with your two hand cards at either end. That should be unbeatable. In fact, even without the turn card, it should still be unbeatable, because nobody else should possibly be able to get anything above four of a kind.