I leave it open. It's not like the W520 isn't bugged and you can connect a 3rd external screen when you close the lid... sadly.
With lid open, there is also more cooling, since the top side can freely radiate heat to the surroundings, instead of having to go through the lid.
The fingerprintreader is also only accessible with open lid.
Sometimes I use an external keyboard, sometimes not, depends on what I'm using it for.
At one place I have a flexible attachment to the desk;

That thing my laptop is standing on, is hinged to the right support post. It can rotate around a vertical axis close to that post. As you can see, I either have to look in the corners of my eyes, turn my neck or my chair. But the TP screen is peanuts compared to external display real estate anyway. It's nice to show some forum and Outlook, or a specsheet, whatever. The main action happens on the 30".
The pic is old; taken before cabling and before I bought the docking station. Even before decent lighting was installed one might say
I have a nicer (especially more sturdy) desk elsewhere, with another dock.
Another big reason to leave the lid open is to keep it from messing up with monitor orientations and resolutions. As you can, I have a 20-30-20 PLP setup; only two are connected to the Thinkpad Dock; the 20" is in portrait mode. If the TP has it's way, it wants to do all kinds of crazy things. Sometimes with sleep / wake cycles in there, I need to undock/dock it to regain correct monitor settings. I have absolutely zero interest in adding opening / closing the lid to that. But I guess if you have the lid closed all the time, it works perfectly fine.
Now if only it worked well with color profiles and external screens... sigh. It often applies the ICC profile of the TP display to my main external monitor; making both ugly. (too yellow on the 30", too blue/green on the TP). Standalone it properly applies the TP ICC profile to the TP. Even though I have calibrated the external screens as well; it simply does not properly apply the profiles all the time.