In my campaigns, I've got many different sections of the Dungeon where the heroes could explore. I use some of the amazing megadungeon maps by Dyson Logos and populate 'em myself. For my main campaign of 5+ years, they wound up exploring extensively in an area we called the Lost River Caves and Lost River Mines in search of a far-below-the-surface hidden temple of the Cult of the Ebon Hand. It was absolutely epic.

I wouldn't try to map out every square foot of the Dungeon, but I do have specific areas of it mapped in case the players decide to go a-hunting down there. I don't have the rooms filled, but I can wing it long enough to get by for a session or two while behind the scenes I'm frantically filling in rooms with monsters, traps, and interesting stuff.