Um, wrong.
Generally, a campaign hums along with encounters geared to what the PC's can handle (more or less) but suppose there are tales of "The Great Dragon of Foofle Peak"? TRUE stories abound of the dragon laying waste to kingdoms and armies and having reaped 1/4 of the worlds treasure all for himself. If the PC's go to Foofle Peak when they are 10th level the level of threat being presented by The Great Dragon is NOT irrelevant. A Great Dragon geared to 10th level PC's is not necessarily all that Great if those tales are SUPPOSED to be true. Certainly, if the PC's are only 3rd level and decide that they can take on The Great Dragon they should simply be brushed aside and the players told, "How STUPID are you? Roll up new characters."
Generally the threat being presented to the PC's should be geared to the PC's level. Why? Because it's BORING if every encounter is a TPK or a cakewalk. It's more fun when encounters truly are challenging, even VERY challenging. But level of threat is never, EVER EVER irrelevant and should not ALWAYS be geared with exactitude to what the PC's can handle.