The supporting classification is sometimes tough to nail down, but it largely a matter of screen time, not the importance of the character.

Waltz disappeared from large stretches of the story, including the entire second and fourth chapters. Leads are commonly considered characters who dominate the story - think Mickey Rourke in The Wrestler. I think Basterds would be correctly classified as an ensemble without a lead performance like Gosford Park or Magnolia.

You could make the case that the Oscars fudge the boundaries on this all the time and you'd be right. This often has more to do with star power and maneuvering than accurately reflecting the films. Tom Cruise was placed in lead over Jamie Foxx for Collateral even though Foxx was in every seen of the movie. Likewise Brad Pitt was pushed as lead for Basterds despite having no more screen time than Waltz. This is because in both cases it makes sense to push the bigger star because he stands a better chance in lead and because it increases both of their chances if they're not in competition with each other.

Sometimes it really is impossible to say. I think you could make a case either way for Heath Ledger in the Dark Knight, for example.