I never look at the PCs before determining which monsters live in an area, or how those monsters are organized. At most, I'll keep in mind the concept of level advancement when I design a world, such that characters might progress through successively stronger areas as they move further away from civilization.
If I were to change the monsters to keep them in line with the PCs, then it would result in degenerate gameplay, such as a fighter who doesn't use armor or weapons because they know that there's no point. I would rather that the players make decisions as their characters would, which means they should optimize themselves to the best of their abilities, because they know that the world isn't going to go easy on them just because they're incompetent.
I find this to be true and not true for me.
What I mean is, for example, I have wandering encounter charts for my world. I also have every monster charted for its geographical location/habitat in my homebrew world (example, basilisks are native to a single mountain range in my world). So in that sense, no, I do not change these encounters based on PC level or power.
HOWEVER, a random roll of, say, an adult red dragon when the PCs are at 1st level is going to be an entirely different encounter than one when the PCs are 12th level. The former will be a far off sighting where the PCs have the chance to take cover, or the dragon might even want something from the PCs and make peaceful contact. The latter could be a straight up fight.
That said, my PCs know if they have fair warning, then it's on.
As an aside, in a 2e campaign I once put 1st level PCs up against a demilich (in a fashion). They were tasked with recovering an item from an ancient wizard's tower. They recovered the item on the second to last upper level of the tower after fighting level appropriate foes. Being curious adventurers, they went to the top floor and found an ornate sarcophagus, carved with arcane runes of power. A PC touched the sarcophagus and a floating skull with gems for eyes and teeth phased through it and told them that it was the demilich spirit of the wizard and if they touched or disturbed the sarcophagus in any way, it would consume their souls. All but one of the PCs fled. The last PLAYER said "There is no way this is a demilich. That's the most powerful creature in 2e and Devin would never throw that at 1st level characters". So her PC touched it, the skull floated up, and consumed the PCs' soul. End of PC.
My point is that you can keep verisimilitude in a setting and not scale all encounters to the PCs while still accommodating their level or lack thereof.