Do you need to...not really, but they do tend to need to cool themselves more often. Personally, we never shut off our desktop, it runs 24/7, as for my notebook, I just use sleep mode on lid shut.
I just close mine, too, but I turn it upside down so the vents aren't against the table.
what really happens when you leave your computer on at night:
http://www.xs4all.nl/~jvdkuyp/flash/see.htm

If something is scheduled to record (we use WMP as our DVR/cable connection) the desktop stays on. We have everything plugged into power strips, those strips are off at night.
Seriously all of you? I'm not any sort of tech expert, but I often troube shoot software problems for customers anyway because of related services and thats the problem 75% of the time. My friend is an admin assistant for the tech department of a large comany and she said its more like 90% for employees that bring in their iphones.Your computer, various programs and iphones schedule various tasks for when they are shut down and when they are turned on. They are designed to be shut down when not in use.