I just read someone say that wandering monster tables that don't "deplete" are being unrealistic. The idea: if adventurers in a dungeon are clearing out a level, then eventually there should be no wandering monster rolls, unless you are cheating and having monster appear from nowhere. (As one person put it, they pop in from "the Elemental Plane of Random Encounters". Which actually is kind of an interesting idea...)
But wait a minute. Are dungeons closed systems?
The adventurers weren't born in the dungeon (usually.) They came in from outside. Who's to say that other things can't enter the dungeon? Certain entries on the monster table should deplete, but some things, especially vermin, should be more or less endless, unless the party takes steps to block them. Bringing in house cats to patrol the edges of a dungeon may eventually eliminate normal rat encounters (not giant rats, though; you'll need dogs for that.) A unique spell like Protection from Bats may prevent restocking of bats.
Most of the time, "empty" slots on your depleted wandering monster table should be replaced with an adjacent entry on the table. When you clear out the orcs, "Orc" results will be replaced with something else, usually vermin. However, if you roll the same number three times in a row, you should probably restock that slot with a new monster from outside the dungeon. At the very least, there should be a 1 in 6 chance every day that an empty slot should get filled with a new creature. Most of the time, this will be a wild animal from above ground, or vermin from an adjacent area. Every once in a while, it will be another party of adventurers, or human bandits or the like, or humanoids that have traveled a long ways to find a new home.