If you know where the drain is on the tank, it would not hurt to lean slightly in that direction...but level is fine as well. Actually, most anything reasonably level is okay, too; it's not as though it's the end of the world if a miniscule amount remains in the tank after draining. Sometimes we all tend to make these things rather more complicated than they need to be.
I'd say level or leaning toward the drain side but more than that, I would say as soon as you can after driving. That way everything is mixed up. Kind of like a bottle of orange juice, shook up to mix in the "pulp". ;)