As a DM, it really depends on the dungeon.
My dungeon's are highly themed - if you are in a goblin lair, you are pretty much only going to face goblinoids.
Since my party has mostly been facing goblinoids, the monsters have averaged out about the party level. That said, when they face something unusual (they fought a dragon at one point), that something unusual tends to be higher level.
The "unusual" encounters, however, are chosen because of what I want to happen plot wise, though, not because I think "hey... let's throw in a dragon to make things different!"
Inquisitor Psychologis Ruminahui