I strongly disagree. If the world conditions itself on the party, that's not a sandbox.
If the party goes down to level X of the dungeon (or equivalent), they're going to find level X monsters there. They won't suddenly become level II monsters just because it's a second-level party. The PCs choose their own level of risk/reward.
I'm pretty sure Mr. Gygax would have said no, but sandbox may be a modern term and it can depart from his advice.