I always mark. Whenever I join a dungeon I always ask what happened to their previous tank, their style, and if they did anything which the group didn't like so I can avoid the same mistake. I have yet to replace a tank that DID mark targets, and my group always seem to sing praises when I tell them that I DO mark targets.

Just get into the habit, it's not difficult and it makes pulls a lot easier. Is it really worth a few extra seconds shaved off your dungeon timer when mobs are scattered everywhere and your healer is losing MP from having to heal more than just one person? For the sake of the healer's brain, I prefer to burn one mob at a time.

It takes longer to raise a fallen ally or retry a pull from aggro flying all over the place than it does taking the targets one at a time.