It isn't about them having time to prepare and think about it. It is about whether I am railroading them to them inn.
So so your parties find dungeons in ways other than: (a) happening to trigger pre-planned conditions to give them information about the dungeon, (b) having sought out information about places of interest, or (c) bumbling across them in there preset location on the map?
What is the biggest difference between having (a) and having a DM not an where the dungeons.fo.in advance and putting it in a he's the party is travelling through but allowing them to bypass it if desires.
Changing them by party level is a preference issue. I personally don't. I like having threats that can be all over the map in terms of power levels. I never cared for dungeon levels being keyed in that way either.
Following up with (c) above as well as random.encounters, how do you avoid a lot of low level TPKs? Do the dungeons appropriate for the high level characters always telegraph they're going to be high level, not have traps appropriate for high level parties, and not have monsters that pursue flee-ers? Do the high level random tables not include predators that would just eat a low level party?