The only problem I've heard of is the possible damage to the screen because it sits right above the video card. I use an external monitor too, and I keep the lid slightly ajar for ventilation. It's also helpful for accessing the power button,
I ran mine with the lid shut on a external monitor a few weeks ago, it was ok for about an hour or 2 but then extreme artifacts came up on screen, I shut down and left it for about 30mins to cool down then it was ok, from now on I always leave the lid ajar when running on a external monitor.
Ya, I would suggest keeping a crack on it too . You will be praying to the if you ever end up with problems. I have posted somewere before, I ran the airconditioning vent directly to the lappy, not because I was having problems, because the keyboard was getting hot