Well, somewhat. With the way healing works in 5E it’s pretty easy to get people back up, and you can often have one PC be downed and still have the rest of the party doing pretty well. If I’m running an enemy that wants to kill the entire party, and the party is trying to kill the enemy, having one at least person go unconscious is pretty common to make it actually feel challenging. But that doesn’t necessarily mean that the party is actually in danger of losing the fight.
Yup, always go higher than the book says, especially with a high-level party. Tier 4 PCs have so much stuff they can do it’s really hard to challenge them, so you gotta get nuts.